Export of internal Abseil changes

--
f6fbb03bff276e72123e8590519079e87732ae62 by Abseil Team <absl-team@google.com>:

Replace static absl::Mutex with SpinLock in absl::Cords

to avoid static initializers by absl::Mutex destructors

PiperOrigin-RevId: 370694199

--
654b7d9edfdc24f226990b2b46cbf91451a1d92a by Martijn Vels <mvels@google.com>:

Implement global data for CordzInfo in an ODR hardened way

This change puts the global data into a global list structure, and stores a reference to the global list in the handle itself. This hardens the implementation against ODR violations where info pointers are crossing dynamic library boundaries which are privately loaded.

PiperOrigin-RevId: 370673045

--
712dba768e66ee2ba85d6010829c617cd2af6ba7 by Martijn Vels <mvels@google.com>:

Intrument Cord::operator= for Cordz

PiperOrigin-RevId: 370659149

--
c0b347a2289e151b72680269332e264b8fa989c0 by Matt Kulukundis <kfm@google.com>:

Fix test guards for ABSL_ATTRIBUTE_RETURNS_NONNULL

PiperOrigin-RevId: 370594807

--
c2bedaa3472ef223f907de2604f9b9b58852ec5f by Martijn Vels <mvels@google.com>:

Add new Cordz instrumentation on GetAppendRegion.

PiperOrigin-RevId: 370587761

--
84fbfcc852697d509f6094482b86e84743a6b331 by Martijn Vels <mvels@google.com>:

Add instrumentation on Cord::Apppend(string_view)

PiperOrigin-RevId: 370576590

--
9e077390b8ca2239e1cb7bfbe1d5a04f2fc11d30 by Abseil Team <absl-team@google.com>:

Google-internal changes only.

PiperOrigin-RevId: 370558424

--
fb53c149eb2364ea34e3a67235f873866618b8ac by Matt Kulukundis <kfm@google.com>:

Update config.h macros with a few useful helpers to simplify version checking

PiperOrigin-RevId: 370557684

--
abf8142e99b9ff7e15f6528a357f1005461950b0 by Martijn Vels <mvels@google.com>:

clang-format cord

PiperOrigin-RevId: 370549371

--
e555985eabe63fcf0e980e9c433dd84caffec191 by Martijn Vels <mvels@google.com>:

Add MaybeUntrackCord() function

This function is near identical to the old UntrackCord() but allows info to be null, moving the cord.is_profiled() branch into CordzInfo.

PiperOrigin-RevId: 370528447

--
3883538efe4601f7864bda70a50d868bb383c63b by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 370503186

--
a9514b65542fde1bc73584e6f3c1c4b3a05f215f by Derek Mauro <dmauro@google.com>:

Add -Winvalid-constexpr to warning options for LLVM

PiperOrigin-RevId: 370455171

--
d8a3966de2cf15a2dc28e17e49a3d27d205eca92 by Martijn Vels <mvels@google.com>:

Add naive UniqueGenerator<T, kMaxValues, ...> to avoid flakes from dup random values.

PiperOrigin-RevId: 370179772

--
46d0caa1a12b68a5998d4f919e20f0f83b9286f8 by Martijn Vels <mvels@google.com>:

Add new Cordz instrumentation on PrependTree.

PiperOrigin-RevId: 370138969
GitOrigin-RevId: f6fbb03bff276e72123e8590519079e87732ae62
Change-Id: Ifa4c00a5c7b01198ee367a3253bea6b66612135e

14 files changed, 461 insertions, 288 deletions

diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 5d4ad7e3..68c71f0a 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -354,6 +354,7 @@ cc_library(
     hdrs = ["internal/cordz_handle.h"],
     copts = ABSL_DEFAULT_COPTS,
     deps = [
+        "//absl/base",
         "//absl/base:config",
         "//absl/base:raw_logging_internal",
         "//absl/synchronization",
@@ -371,8 +372,10 @@ cc_library(
         ":cordz_handle",
         ":cordz_statistics",
         ":cordz_update_tracker",
+        "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/debugging:stacktrace",
         "//absl/synchronization",
         "//absl/types:span",
@@ -525,6 +528,7 @@ cc_library(
     deps = [
         ":cord",
         ":cord_internal",
+        ":strings",
     ],
 )
 
@@ -587,6 +591,7 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":cord",
+        ":cord_test_helpers",
         ":cordz_functions",
         ":cordz_info",
         ":cordz_sample_token",
diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index 2a8e3df2..80ae2a60 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -655,6 +655,7 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base
     absl::config
     absl::raw_logging_internal
     absl::synchronization
@@ -687,6 +688,7 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base
     absl::config
     absl::cord_internal
     absl::cordz_functions
@@ -695,6 +697,7 @@ absl_cc_library(
     absl::cordz_update_tracker
     absl::core_headers
     absl::span
+    absl::raw_logging_internal
     absl::stacktrace
     absl::synchronization
 )
@@ -829,6 +832,7 @@ absl_cc_library(
   DEPS
     absl::cord
     absl::cord_internal
+    absl::strings
   TESTONLY
 )
 
@@ -914,6 +918,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::cord
+    absl::cord_test_helpers
     absl::cordz_test_helpers
     absl::cordz_functions
     absl::cordz_info
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index 768106d4..15d17337 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -210,7 +210,7 @@ static CordRep* MakeBalancedTree(CordRep** reps, size_t n) {
 }
 
 static CordRepFlat* CreateFlat(const char* data, size_t length,
-                            size_t alloc_hint) {
+                               size_t alloc_hint) {
   CordRepFlat* flat = CordRepFlat::New(length + alloc_hint);
   flat->length = length;
   memcpy(flat->Data(), data, length);
@@ -234,9 +234,7 @@ static CordRep* RingNewTree(const char* data, size_t length,
 
 // Create a new tree out of the specified array.
 // The returned node has a refcount of 1.
-static CordRep* NewTree(const char* data,
-                        size_t length,
-                        size_t alloc_hint) {
+static CordRep* NewTree(const char* data, size_t length, size_t alloc_hint) {
   if (length == 0) return nullptr;
   if (cord_ring_enabled()) {
     return RingNewTree(data, length, alloc_hint);
@@ -303,24 +301,6 @@ inline char* Cord::InlineRep::set_data(size_t n) {
   return data_.as_chars();
 }
 
-static inline CordRepFlat* MakeFlat(const InlineData& data, size_t extra = 0) {
-  static_assert(kMinFlatLength >= sizeof(data), "");
-  size_t len = data.inline_size();
-  CordRepFlat* result = CordRepFlat::New(len + extra);
-  result->length = len;
-  memcpy(result->Data(), data.as_chars(), sizeof(data));
-  return result;
-}
-
-inline CordRep* Cord::InlineRep::force_tree(size_t extra_hint) {
-  if (data_.is_tree()) {
-    return data_.as_tree();
-  }
-  CordRepFlat* result = MakeFlat(data_, extra_hint);
-  set_tree(result);
-  return result;
-}
-
 inline void Cord::InlineRep::reduce_size(size_t n) {
   size_t tag = inline_size();
   assert(tag <= kMaxInline);
@@ -346,7 +326,7 @@ void Cord::InlineRep::AppendTreeToInlined(CordRep* tree,
                                           MethodIdentifier method) {
   assert(!is_tree());
   if (!data_.is_empty()) {
-    CordRepFlat* flat = MakeFlat(data_);
+    CordRepFlat* flat = MakeFlatWithExtraCapacity(0);
     if (cord_ring_enabled()) {
       tree = CordRepRing::Append(CordRepRing::Create(flat, 1), tree);
     } else {
@@ -376,14 +356,38 @@ void Cord::InlineRep::AppendTree(CordRep* tree, MethodIdentifier method) {
   }
 }
 
-void Cord::InlineRep::PrependTree(CordRep* tree) {
+void Cord::InlineRep::PrependTreeToInlined(CordRep* tree,
+                                           MethodIdentifier method) {
+  assert(!is_tree());
+  if (!data_.is_empty()) {
+    CordRepFlat* flat = MakeFlatWithExtraCapacity(0);
+    if (cord_ring_enabled()) {
+      tree = CordRepRing::Prepend(CordRepRing::Create(flat, 1), tree);
+    } else {
+      tree = Concat(tree, flat);
+    }
+  }
+  EmplaceTree(tree, method);
+}
+
+void Cord::InlineRep::PrependTreeToTree(CordRep* tree,
+                                        MethodIdentifier method) {
+  assert(is_tree());
+  const CordzUpdateScope scope(data_.cordz_info(), method);
+  if (cord_ring_enabled()) {
+    tree = CordRepRing::Prepend(ForceRing(data_.as_tree(), 1), tree);
+  } else {
+    tree = Concat(tree, data_.as_tree());
+  }
+  SetTree(tree, scope);
+}
+
+void Cord::InlineRep::PrependTree(CordRep* tree, MethodIdentifier method) {
   assert(tree != nullptr);
-  if (data_.is_empty()) {
-    set_tree(tree);
-  } else if (cord_ring_enabled()) {
-    set_tree(CordRepRing::Prepend(ForceRing(force_tree(0), 1), tree));
+  if (data_.is_tree()) {
+    PrependTreeToTree(tree, method);
   } else {
-    set_tree(Concat(tree, force_tree(0)));
+    PrependTreeToInlined(tree, method);
   }
 }
 
@@ -435,78 +439,43 @@ static inline bool PrepareAppendRegion(CordRep* root, char** region,
   return true;
 }
 
+template <bool has_length>
 void Cord::InlineRep::GetAppendRegion(char** region, size_t* size,
-                                      size_t max_length) {
-  if (max_length == 0) {
-    *region = nullptr;
-    *size = 0;
-    return;
-  }
-
-  // Try to fit in the inline buffer if possible.
-  if (!is_tree()) {
-    size_t inline_length = inline_size();
-    if (max_length <= kMaxInline - inline_length) {
-      *region = data_.as_chars() + inline_length;
-      *size = max_length;
-      set_inline_size(inline_length + max_length);
+                                      size_t length) {
+  auto constexpr method = CordzUpdateTracker::kGetAppendRegion;
+
+  CordRep* root = tree();
+  size_t sz = root ? root->length : inline_size();
+  if (root == nullptr) {
+    size_t available = kMaxInline - sz;
+    if (available >= (has_length ? length : 1)) {
+      *region = data_.as_chars() + sz;
+      *size = has_length ? length : available;
+      set_inline_size(has_length ? sz + length : kMaxInline);
       return;
     }
   }
 
-  CordRep* root = force_tree(max_length);
-
-  if (PrepareAppendRegion(root, region, size, max_length)) {
-    UpdateCordzStatistics();
+  size_t extra = has_length ? length : (std::max)(sz, kMinFlatLength);
+  CordRep* rep = root ? root : MakeFlatWithExtraCapacity(extra);
+  CordzUpdateScope scope(root ? data_.cordz_info() : nullptr, method);
+  if (PrepareAppendRegion(rep, region, size, length)) {
+    CommitTree(root, rep, scope, method);
     return;
   }
 
   // Allocate new node.
-  CordRepFlat* new_node =
-      CordRepFlat::New(std::max(static_cast<size_t>(root->length), max_length));
-  new_node->length = std::min(new_node->Capacity(), max_length);
+  CordRepFlat* new_node = CordRepFlat::New(extra);
+  new_node->length = std::min(new_node->Capacity(), length);
   *region = new_node->Data();
   *size = new_node->length;
 
   if (cord_ring_enabled()) {
-    replace_tree(CordRepRing::Append(ForceRing(root, 1), new_node));
-    return;
-  }
-  replace_tree(Concat(root, new_node));
-}
-
-void Cord::InlineRep::GetAppendRegion(char** region, size_t* size) {
-  const size_t max_length = std::numeric_limits<size_t>::max();
-
-  // Try to fit in the inline buffer if possible.
-  if (!data_.is_tree()) {
-    size_t inline_length = inline_size();
-    if (inline_length < kMaxInline) {
-      *region = data_.as_chars() + inline_length;
-      *size = kMaxInline - inline_length;
-      set_inline_size(kMaxInline);
-      return;
-    }
-  }
-
-  CordRep* root = force_tree(max_length);
-
-  if (PrepareAppendRegion(root, region, size, max_length)) {
-    UpdateCordzStatistics();
-    return;
-  }
-
-  // Allocate new node.
-  CordRepFlat* new_node = CordRepFlat::New(root->length);
-  new_node->length = new_node->Capacity();
-  *region = new_node->Data();
-  *size = new_node->length;
-
-  if (cord_ring_enabled()) {
-    replace_tree(CordRepRing::Append(ForceRing(root, 1), new_node));
-    return;
+    rep = CordRepRing::Append(ForceRing(rep, 1), new_node);
+  } else {
+    rep = Concat(rep, new_node);
   }
-  replace_tree(Concat(root, new_node));
+  CommitTree(root, rep, scope, method);
 }
 
 // If the rep is a leaf, this will increment the value at total_mem_usage and
@@ -529,21 +498,26 @@ void Cord::InlineRep::UpdateCordzStatisticsSlow() {
 }
 
 void Cord::InlineRep::AssignSlow(const Cord::InlineRep& src) {
-  UnrefTree();
-
-  data_ = src.data_;
-  if (is_tree()) {
-    CordRep::Ref(tree());
-    clear_cordz_info();
-    CordzInfo::MaybeTrackCord(data_, CordzUpdateTracker::kAssignCord);
+  assert(&src != this);
+  assert(is_tree() || src.is_tree());
+  auto constexpr method = CordzUpdateTracker::kAssignCord;
+  if (CordRep* tree = this->tree()) {
+    CordzUpdateScope scope(data_.cordz_info(), method);
+    CordRep::Unref(tree);
+    if (CordRep* src_tree = src.tree()) {
+      SetTree(CordRep::Ref(src_tree), scope);
+    } else {
+      scope.SetCordRep(nullptr);
+      data_ = src.data_;
+    }
+  } else {
+    EmplaceTree(CordRep::Ref(src.as_tree()), method);
   }
 }
 
 void Cord::InlineRep::UnrefTree() {
   if (is_tree()) {
-    if (ABSL_PREDICT_FALSE(is_profiled())) {
-      absl::cord_internal::CordzInfo::UntrackCord(cordz_info());
-    }
+    CordzInfo::MaybeUntrackCord(data_.cordz_info());
     CordRep::Unref(tree());
   }
 }
@@ -595,12 +569,9 @@ template Cord::Cord(std::string&& src);
 // The destruction code is separate so that the compiler can determine
 // that it does not need to call the destructor on a moved-from Cord.
 void Cord::DestroyCordSlow() {
-  if (ABSL_PREDICT_FALSE(contents_.is_profiled())) {
-    absl::cord_internal::CordzInfo::UntrackCord(contents_.cordz_info());
-  }
-  if (CordRep* tree = contents_.tree()) {
-    CordRep::Unref(VerifyTree(tree));
-  }
+  assert(contents_.is_tree());
+  CordzInfo::MaybeUntrackCord(contents_.cordz_info());
+  CordRep::Unref(VerifyTree(contents_.as_tree()));
 }
 
 // --------------------------------------------------------------------
@@ -613,23 +584,18 @@ void Cord::Clear() {
 }
 
 Cord& Cord::operator=(absl::string_view src) {
-  if (ABSL_PREDICT_FALSE(contents_.is_profiled())) {
-    absl::cord_internal::CordzInfo::UntrackCord(contents_.cordz_info());
-    contents_.clear_cordz_info();
-  }
-
   const char* data = src.data();
   size_t length = src.size();
   CordRep* tree = contents_.tree();
   if (length <= InlineRep::kMaxInline) {
     // Embed into this->contents_
+    if (tree) CordzInfo::MaybeUntrackCord(contents_.cordz_info());
     contents_.set_data(data, length, true);
     if (tree) CordRep::Unref(tree);
     return *this;
   }
   if (tree != nullptr && tree->tag >= FLAT &&
-      tree->flat()->Capacity() >= length &&
-      tree->refcount.IsOne()) {
+      tree->flat()->Capacity() >= length && tree->refcount.IsOne()) {
     // Copy in place if the existing FLAT node is reusable.
     memmove(tree->flat()->Data(), data, length);
     tree->length = length;
@@ -655,71 +621,70 @@ template Cord& Cord::operator=(std::string&& src);
 
 // TODO(sanjay): Move to Cord::InlineRep section of file.  For now,
 // we keep it here to make diffs easier.
-void Cord::InlineRep::AppendArray(const char* src_data, size_t src_size) {
-  if (src_size == 0) return;  // memcpy(_, nullptr, 0) is undefined.
+void Cord::InlineRep::AppendArray(absl::string_view src,
+                                  MethodIdentifier method) {
+  if (src.empty()) return;  // memcpy(_, nullptr, 0) is undefined.
 
   size_t appended = 0;
-  CordRep* root = nullptr;
-  if (is_tree()) {
-    root = data_.as_tree();
+  CordRep* rep = tree();
+  const CordRep* const root = rep;
+  CordzUpdateScope scope(root ? cordz_info() : nullptr, method);
+  if (root != nullptr) {
     char* region;
-    if (PrepareAppendRegion(root, &region, &appended, src_size)) {
-      memcpy(region, src_data, appended);
-      UpdateCordzStatistics();
+    if (PrepareAppendRegion(rep, &region, &appended, src.size())) {
+      memcpy(region, src.data(), appended);
     }
   } else {
     // Try to fit in the inline buffer if possible.
     size_t inline_length = inline_size();
-    if (src_size <= kMaxInline - inline_length) {
+    if (src.size() <= kMaxInline - inline_length) {
       // Append new data to embedded array
-      memcpy(data_.as_chars() + inline_length, src_data, src_size);
-      set_inline_size(inline_length + src_size);
+      memcpy(data_.as_chars() + inline_length, src.data(), src.size());
+      set_inline_size(inline_length + src.size());
       return;
     }
 
-    // It is possible that src_data == data_, but when we transition from an
+    // It is possible that src.data() == data_, but when we transition from an
     // InlineRep to a tree we need to assign data_ = root via set_tree. To
     // avoid corrupting the source data before we copy it, delay calling
     // set_tree until after we've copied data.
     // We are going from an inline size to beyond inline size. Make the new size
     // either double the inlined size, or the added size + 10%.
-    const size_t size1 = inline_length * 2 + src_size;
-    const size_t size2 = inline_length + src_size / 10;
-    root = CordRepFlat::New(std::max<size_t>(size1, size2));
-    appended = std::min(
-        src_size, root->flat()->Capacity() - inline_length);
-    memcpy(root->flat()->Data(), data_.as_chars(), inline_length);
-    memcpy(root->flat()->Data() + inline_length, src_data, appended);
-    root->length = inline_length + appended;
-    set_tree(root);
-  }
-
-  src_data += appended;
-  src_size -= appended;
-  if (src_size == 0) {
+    const size_t size1 = inline_length * 2 + src.size();
+    const size_t size2 = inline_length + src.size() / 10;
+    rep = CordRepFlat::New(std::max<size_t>(size1, size2));
+    appended = std::min(src.size(), rep->flat()->Capacity() - inline_length);
+    memcpy(rep->flat()->Data(), data_.as_chars(), inline_length);
+    memcpy(rep->flat()->Data() + inline_length, src.data(), appended);
+    rep->length = inline_length + appended;
+  }
+
+  src.remove_prefix(appended);
+  if (src.empty()) {
+    CommitTree(root, rep, scope, method);
     return;
   }
 
   if (cord_ring_enabled()) {
-    absl::string_view data(src_data, src_size);
-    root = ForceRing(root, (data.size() - 1) / kMaxFlatLength + 1);
-    replace_tree(CordRepRing::Append(root->ring(), data));
-    return;
-  }
-
-  // Use new block(s) for any remaining bytes that were not handled above.
-  // Alloc extra memory only if the right child of the root of the new tree is
-  // going to be a FLAT node, which will permit further inplace appends.
-  size_t length = src_size;
-  if (src_size < kMaxFlatLength) {
-    // The new length is either
-    // - old size + 10%
-    // - old_size + src_size
-    // This will cause a reasonable conservative step-up in size that is still
-    // large enough to avoid excessive amounts of small fragments being added.
-    length = std::max<size_t>(root->length / 10, src_size);
+    rep = ForceRing(rep, (src.size() - 1) / kMaxFlatLength + 1);
+    rep = CordRepRing::Append(rep->ring(), src);
+  } else {
+    // Use new block(s) for any remaining bytes that were not handled above.
+    // Alloc extra memory only if the right child of the root of the new tree
+    // is going to be a FLAT node, which will permit further inplace appends.
+    size_t length = src.size();
+    if (src.size() < kMaxFlatLength) {
+      // The new length is either
+      // - old size + 10%
+      // - old_size + src.size()
+      // This will cause a reasonable conservative step-up in size that is
+      // still large enough to avoid excessive amounts of small fragments
+      // being added.
+      length = std::max<size_t>(rep->length / 10, src.size());
+    }
+    rep = Concat(rep, NewTree(src.data(), src.size(), length - src.size()));
   }
-  set_tree(Concat(root, NewTree(src_data, src_size, length - src_size)));
+  CommitTree(root, rep, scope, method);
 }
 
 inline CordRep* Cord::TakeRep() const& {
@@ -734,12 +699,13 @@ inline CordRep* Cord::TakeRep() && {
 
 template <typename C>
 inline void Cord::AppendImpl(C&& src) {
+  auto constexpr method = CordzUpdateTracker::kAppendCord;
   if (empty()) {
     // Since destination is empty, we can avoid allocating a node,
     if (src.contents_.is_tree()) {
       // by taking the tree directly
       CordRep* rep = std::forward<C>(src).TakeRep();
-      contents_.EmplaceTree(rep, CordzUpdateTracker::kAppendCord);
+      contents_.EmplaceTree(rep, method);
     } else {
       // or copying over inline data
       contents_.data_ = src.contents_.data_;
@@ -753,12 +719,12 @@ inline void Cord::AppendImpl(C&& src) {
     CordRep* src_tree = src.contents_.tree();
     if (src_tree == nullptr) {
       // src has embedded data.
-      contents_.AppendArray(src.contents_.data(), src_size);
+      contents_.AppendArray({src.contents_.data(), src_size}, method);
       return;
     }
     if (src_tree->tag >= FLAT) {
       // src tree just has one flat node.
-      contents_.AppendArray(src_tree->flat()->Data(), src_size);
+      contents_.AppendArray({src_tree->flat()->Data(), src_size}, method);
       return;
     }
     if (&src == this) {
@@ -797,7 +763,7 @@ void Cord::Prepend(const Cord& src) {
   CordRep* src_tree = src.contents_.tree();
   if (src_tree != nullptr) {
     CordRep::Ref(src_tree);
-    contents_.PrependTree(src_tree);
+    contents_.PrependTree(src_tree, CordzUpdateTracker::kPrependCord);
     return;
   }
 
@@ -820,7 +786,8 @@ void Cord::Prepend(absl::string_view src) {
       return;
     }
   }
-  contents_.PrependTree(NewTree(src.data(), src.size(), 0));
+  CordRep* rep = NewTree(src.data(), src.size(), 0);
+  contents_.PrependTree(rep, CordzUpdateTracker::kPrependString);
 }
 
 template <typename T, Cord::EnableIfString<T>>
@@ -1847,8 +1814,7 @@ static void DumpNode(CordRep* rep, bool include_data, std::ostream* os,
           *os << absl::CEscape(std::string(rep->external()->base, rep->length));
         *os << "]\n";
       } else if (rep->tag >= FLAT) {
-        *os << "FLAT cap=" << rep->flat()->Capacity()
-            << " [";
+        *os << "FLAT cap=" << rep->flat()->Capacity() << " [";
         if (include_data)
           *os << absl::CEscape(std::string(rep->flat()->Data(), rep->length));
         *os << "]\n";
@@ -1860,7 +1826,7 @@ static void DumpNode(CordRep* rep, bool include_data, std::ostream* os,
         do {
           DumpNode(ring->entry_child(head), include_data, os,
                    indent + kIndentStep);
-          head = ring->advance(head);;
+          head = ring->advance(head);
         } while (head != ring->tail());
       }
       if (stack.empty()) break;
@@ -1906,9 +1872,8 @@ static bool VerifyNode(CordRep* root, CordRep* start_node,
         worklist.push_back(node->concat()->left);
       }
     } else if (node->tag >= FLAT) {
-      ABSL_INTERNAL_CHECK(
-          node->length <= node->flat()->Capacity(),
-          ReportError(root, node));
+      ABSL_INTERNAL_CHECK(node->length <= node->flat()->Capacity(),
+                          ReportError(root, node));
     } else if (node->tag == EXTERNAL) {
       ABSL_INTERNAL_CHECK(node->external()->base != nullptr,
                           ReportError(root, node));
diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index dd6c3bca..d5a13b34 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h
@@ -671,6 +671,7 @@ class Cord {
 
  private:
   using CordRep = absl::cord_internal::CordRep;
+  using CordRepFlat = absl::cord_internal::CordRepFlat;
   using CordzInfo = cord_internal::CordzInfo;
   using CordzUpdateScope = cord_internal::CordzUpdateScope;
   using CordzUpdateTracker = cord_internal::CordzUpdateTracker;
@@ -728,12 +729,15 @@ class Cord {
     // Returns non-null iff was holding a pointer
     absl::cord_internal::CordRep* clear();
     // Converts to pointer if necessary.
-    absl::cord_internal::CordRep* force_tree(size_t extra_hint);
     void reduce_size(size_t n);  // REQUIRES: holding data
     void remove_prefix(size_t n);  // REQUIRES: holding data
-    void AppendArray(const char* src_data, size_t src_size);
+    void AppendArray(absl::string_view src, MethodIdentifier method);
     absl::string_view FindFlatStartPiece() const;
 
+    // Creates a CordRepFlat instance from the current inlined data with `extra'
+    // bytes of desired additional capacity.
+    CordRepFlat* MakeFlatWithExtraCapacity(size_t extra);
+
     // Sets the tree value for this instance. `rep` must not be null.
     // Requires the current instance to hold a tree, and a lock to be held on
     // any CordzInfo referenced by this instance. The latter is enforced through
@@ -747,12 +751,22 @@ class Cord {
     // value to a non-inlined (tree / ring) value.
     void EmplaceTree(CordRep* rep, MethodIdentifier method);
 
+    // Commits the change of a newly created, or updated `rep` root value into
+    // this cord. `old_rep` indicates the old (inlined or tree) value of the
+    // cord, and determines if the commit invokes SetTree() or EmplaceTree().
+    void CommitTree(const CordRep* old_rep, CordRep* rep,
+                    const CordzUpdateScope& scope, MethodIdentifier method);
+
     void AppendTreeToInlined(CordRep* tree, MethodIdentifier method);
     void AppendTreeToTree(CordRep* tree, MethodIdentifier method);
     void AppendTree(CordRep* tree, MethodIdentifier method);
-    void PrependTree(absl::cord_internal::CordRep* tree);
-    void GetAppendRegion(char** region, size_t* size, size_t max_length);
-    void GetAppendRegion(char** region, size_t* size);
+    void PrependTreeToInlined(CordRep* tree, MethodIdentifier method);
+    void PrependTreeToTree(CordRep* tree, MethodIdentifier method);
+    void PrependTree(CordRep* tree, MethodIdentifier method);
+
+    template <bool has_length>
+    void GetAppendRegion(char** region, size_t* size, size_t length);
+
     bool IsSame(const InlineRep& other) const {
       return memcmp(&data_, &other.data_, sizeof(data_)) == 0;
     }
@@ -1041,6 +1055,16 @@ inline size_t Cord::InlineRep::size() const {
   return is_tree() ? as_tree()->length : inline_size();
 }
 
+inline cord_internal::CordRepFlat* Cord::InlineRep::MakeFlatWithExtraCapacity(
+    size_t extra) {
+  static_assert(cord_internal::kMinFlatLength >= sizeof(data_), "");
+  size_t len = data_.inline_size();
+  auto* result = CordRepFlat::New(len + extra);
+  result->length = len;
+  memcpy(result->Data(), data_.as_chars(), sizeof(data_));
+  return result;
+}
+
 inline void Cord::InlineRep::EmplaceTree(CordRep* rep,
                                          MethodIdentifier method) {
   data_.make_tree(rep);
@@ -1055,10 +1079,20 @@ inline void Cord::InlineRep::SetTree(CordRep* rep,
   scope.SetCordRep(rep);
 }
 
+inline void Cord::InlineRep::CommitTree(const CordRep* old_rep, CordRep* rep,
+                                        const CordzUpdateScope& scope,
+                                        MethodIdentifier method) {
+  if (old_rep) {
+    SetTree(rep, scope);
+  } else {
+    EmplaceTree(rep, method);
+  }
+}
+
 inline void Cord::InlineRep::set_tree(absl::cord_internal::CordRep* rep) {
   if (rep == nullptr) {
-    if (ABSL_PREDICT_FALSE(is_profiled())) {
-      absl::cord_internal::CordzInfo::UntrackCord(cordz_info());
+    if (data_.is_tree()) {
+      CordzInfo::MaybeUntrackCord(data_.cordz_info());
     }
     ResetToEmpty();
   } else {
@@ -1086,8 +1120,8 @@ inline void Cord::InlineRep::replace_tree(absl::cord_internal::CordRep* rep) {
 }
 
 inline absl::cord_internal::CordRep* Cord::InlineRep::clear() {
-  if (ABSL_PREDICT_FALSE(is_profiled())) {
-    absl::cord_internal::CordzInfo::UntrackCord(cordz_info());
+  if (is_tree()) {
+    CordzInfo::MaybeUntrackCord(cordz_info());
   }
   absl::cord_internal::CordRep* result = tree();
   ResetToEmpty();
@@ -1180,7 +1214,7 @@ inline absl::string_view Cord::Flatten() {
 }
 
 inline void Cord::Append(absl::string_view src) {
-  contents_.AppendArray(src.data(), src.size());
+  contents_.AppendArray(src, CordzUpdateTracker::kAppendString);
 }
 
 extern template void Cord::Append(std::string&& src);
diff --git a/absl/strings/cord_test_helpers.h b/absl/strings/cord_test_helpers.h
index 3815df81..6ccccc51 100644
--- a/absl/strings/cord_test_helpers.h
+++ b/absl/strings/cord_test_helpers.h
@@ -17,12 +17,50 @@
 #ifndef ABSL_STRINGS_CORD_TEST_HELPERS_H_
 #define ABSL_STRINGS_CORD_TEST_HELPERS_H_
 
+#include <cstdint>
+#include <iostream>
+
 #include "absl/strings/cord.h"
 #include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
+// Cord sizes relevant for testing
+enum class TestCordSize {
+  kEmpty = 0,
+  kInlined = cord_internal::kMaxInline / 2 + 1,
+  kSmall = cord_internal::kMaxBytesToCopy / 2 + 1,
+  kMedium = cord_internal::kMaxFlatLength / 2 + 1,
+  kLarge = cord_internal::kMaxFlatLength * 4
+};
+
+// To string helper
+inline absl::string_view ToString(TestCordSize size) {
+  switch (size) {
+    case TestCordSize::kEmpty:
+      return "Empty";
+    case TestCordSize::kInlined:
+      return "Inlined";
+    case TestCordSize::kSmall:
+      return "Small";
+    case TestCordSize::kMedium:
+      return "Medium";
+    case TestCordSize::kLarge:
+      return "Large";
+  }
+  return "???";
+}
+
+// Returns the length matching the specified size
+inline size_t Length(TestCordSize size) { return static_cast<size_t>(size); }
+
+// Stream output helper
+inline std::ostream& operator<<(std::ostream& stream, TestCordSize size) {
+  return stream << ToString(size);
+}
+
 // Creates a multi-segment Cord from an iterable container of strings.  The
 // resulting Cord is guaranteed to have one segment for every string in the
 // container.  This allows code to be unit tested with multi-segment Cord
diff --git a/absl/strings/cordz_test.cc b/absl/strings/cordz_test.cc
index 6e8deefd..b16e9686 100644
--- a/absl/strings/cordz_test.cc
+++ b/absl/strings/cordz_test.cc
@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <cstdint>
 #include <string>
 
 #include "gmock/gmock.h"
@@ -20,6 +21,7 @@
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
 #include "absl/strings/cord.h"
+#include "absl/strings/cord_test_helpers.h"
 #include "absl/strings/cordz_test_helpers.h"
 #include "absl/strings/internal/cordz_functions.h"
 #include "absl/strings/internal/cordz_info.h"
@@ -31,6 +33,8 @@
 
 #ifdef ABSL_INTERNAL_CORDZ_ENABLED
 
+using testing::Eq;
+
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
@@ -40,48 +44,126 @@ using cord_internal::CordzStatistics;
 using cord_internal::CordzUpdateTracker;
 using Method = CordzUpdateTracker::MethodIdentifier;
 
+// Do not print cord contents, we only care about 'size' perhaps.
+// Note that this method must be inside the named namespace.
+inline void PrintTo(const Cord& cord, std::ostream* s) {
+  if (s) *s << "Cord[" << cord.size() << "]";
+}
+
 namespace {
 
-std::string MakeString(int length) {
-  std::string s(length, '.');
-  for (int i = 4; i < length; i += 2) {
-    s[i] = '\b';
-  }
-  return s;
+// Returns a string_view value of the specified length
+// We do this to avoid 'consuming' large strings in Cord by default.
+absl::string_view MakeString(size_t size) {
+  thread_local std::string str;
+  str = std::string(size, '.');
+  return str;
+}
+
+absl::string_view MakeString(TestCordSize size) {
+  return MakeString(Length(size));
+}
+
+std::string TestParamToString(::testing::TestParamInfo<TestCordSize> size) {
+  return absl::StrCat("On", ToString(size.param), "Cord");
 }
 
-TEST(CordzTest, ConstructSmallStringView) {
-  CordzSamplingIntervalHelper sample_every(1);
-  Cord cord(absl::string_view(MakeString(50)));
+class CordzUpdateTest : public testing::TestWithParam<TestCordSize> {
+ public:
+  Cord& cord() { return cord_; }
+
+  Method InitialOr(Method method) const {
+    return (GetParam() > TestCordSize::kInlined) ? Method::kConstructorString
+                                                 : method;
+  }
+
+ private:
+  CordzSamplingIntervalHelper sample_every_{1};
+  Cord cord_{MakeString(GetParam())};
+};
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordzUpdateTest,
+                         testing::Values(TestCordSize::kEmpty,
+                                         TestCordSize::kInlined,
+                                         TestCordSize::kLarge),
+                         TestParamToString);
+
+TEST(CordzTest, ConstructSmallString) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord(MakeString(TestCordSize::kSmall));
   EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
 }
 
-TEST(CordzTest, ConstructLargeStringView) {
-  CordzSamplingIntervalHelper sample_every(1);
-  Cord cord(absl::string_view(MakeString(5000)));
+TEST(CordzTest, ConstructLargeString) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord(MakeString(TestCordSize::kLarge));
   EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
 }
 
 TEST(CordzTest, CopyConstruct) {
-  CordzSamplingIntervalHelper sample_every(1);
-  Cord src = UnsampledCord(MakeString(5000));
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
   Cord cord(src);
   EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorCord));
 }
 
-TEST(CordzTest, AppendLargeCordToEmpty) {
-  CordzSamplingIntervalHelper sample_every(1);
-  Cord cord;
-  Cord src = UnsampledCord(MakeString(5000));
-  cord.Append(src);
-  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kAppendCord));
+TEST(CordzTest, MoveConstruct) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src(MakeString(TestCordSize::kLarge));
+  Cord cord(std::move(src));
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
 }
 
-TEST(CordzTest, MoveAppendLargeCordToEmpty) {
-  CordzSamplingIntervalHelper sample_every(1);
+TEST_P(CordzUpdateTest, AssignCord) {
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord() = src;
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAssignCord)));
+}
+
+TEST(CordzTest, AssignInlinedCord) {
+  CordzSampleToken token;
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord(MakeString(TestCordSize::kLarge));
+  const CordzInfo* info = GetCordzInfoForTesting(cord);
+  Cord src = UnsampledCord(MakeString(TestCordSize::kInlined));
+  cord = src;
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
+}
+
+TEST(CordzTest, MoveAssignCord) {
+  CordzSamplingIntervalHelper sample_every{1};
   Cord cord;
-  cord.Append(UnsampledCord(MakeString(5000)));
-  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kAppendCord));
+  Cord src(MakeString(TestCordSize::kLarge));
+  cord = std::move(src);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
+}
+
+TEST_P(CordzUpdateTest, AppendCord) {
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord().Append(src);
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendCord)));
+}
+
+TEST_P(CordzUpdateTest, MoveAppendCord) {
+  cord().Append(UnsampledCord(MakeString(TestCordSize::kLarge)));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendCord)));
+}
+
+TEST_P(CordzUpdateTest, PrependCord) {
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord().Prepend(src);
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kPrependCord)));
+}
+
+TEST_P(CordzUpdateTest, AppendSmallArray) {
+  cord().Append(MakeString(TestCordSize::kSmall));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendString)));
+}
+
+TEST_P(CordzUpdateTest, AppendLargeArray) {
+  cord().Append(MakeString(TestCordSize::kLarge));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAppendString)));
 }
 
 }  // namespace
diff --git a/absl/strings/cordz_test_helpers.h b/absl/strings/cordz_test_helpers.h
index 453c2f9d..d9573c97 100644
--- a/absl/strings/cordz_test_helpers.h
+++ b/absl/strings/cordz_test_helpers.h
@@ -15,6 +15,8 @@
 #ifndef ABSL_STRINGS_CORDZ_TEST_HELPERS_H_
 #define ABSL_STRINGS_CORDZ_TEST_HELPERS_H_
 
+#include <utility>
+
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
@@ -32,6 +34,7 @@ ABSL_NAMESPACE_BEGIN
 // Returns the CordzInfo for the cord, or nullptr if the cord is not sampled.
 inline const cord_internal::CordzInfo* GetCordzInfoForTesting(
     const Cord& cord) {
+  if (cord.size() <= cord_internal::kMaxInline) return nullptr;
   return cord.contents_.cordz_info();
 }
 
@@ -44,11 +47,13 @@ inline bool CordzInfoIsListed(const cord_internal::CordzInfo* cordz_info,
   return false;
 }
 
-// Matcher on Cord that verifies all of:
+// Matcher on Cord* that verifies all of:
 // - the cord is sampled
 // - the CordzInfo of the cord is listed / discoverable.
 // - the reported CordzStatistics match the cord's actual properties
 // - the cord has an (initial) UpdateTracker count of 1 for `method`
+// This matcher accepts a const Cord* to avoid having the matcher dump
+// copious amounts of cord data on failures.
 MATCHER_P(HasValidCordzInfoOf, method, "CordzInfo matches cord") {
   const cord_internal::CordzInfo* cord_info = GetCordzInfoForTesting(arg);
   if (cord_info == nullptr) {
diff --git a/absl/strings/internal/cordz_handle.cc b/absl/strings/internal/cordz_handle.cc
index 22d07978..5297ec8e 100644
--- a/absl/strings/internal/cordz_handle.cc
+++ b/absl/strings/internal/cordz_handle.cc
@@ -16,16 +16,19 @@
 #include <atomic>
 
 #include "absl/base/internal/raw_logging.h"  // For ABSL_RAW_CHECK
+#include "absl/base/internal/spinlock.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
+using ::absl::base_internal::SpinLockHolder;
+
 ABSL_CONST_INIT CordzHandle::Queue CordzHandle::global_queue_(absl::kConstInit);
 
 CordzHandle::CordzHandle(bool is_snapshot) : is_snapshot_(is_snapshot) {
   if (is_snapshot) {
-    MutexLock lock(&queue_->mutex);
+    SpinLockHolder lock(&queue_->mutex);
     CordzHandle* dq_tail = queue_->dq_tail.load(std::memory_order_acquire);
     if (dq_tail != nullptr) {
       dq_prev_ = dq_tail;
@@ -40,7 +43,7 @@ CordzHandle::~CordzHandle() {
   if (is_snapshot_) {
     std::vector<CordzHandle*> to_delete;
     {
-      absl::MutexLock lock(&queue_->mutex);
+      SpinLockHolder lock(&queue_->mutex);
       CordzHandle* next = dq_next_;
       if (dq_prev_ == nullptr) {
         // We were head of the queue, delete every CordzHandle until we reach
@@ -70,7 +73,7 @@ void CordzHandle::Delete(CordzHandle* handle) {
     handle->ODRCheck();
     Queue* const queue = handle->queue_;
     if (!handle->is_snapshot_ && !queue->IsEmpty()) {
-      MutexLock lock(&queue->mutex);
+      SpinLockHolder lock(&queue->mutex);
       CordzHandle* dq_tail = queue->dq_tail.load(std::memory_order_acquire);
       if (dq_tail != nullptr) {
         handle->dq_prev_ = dq_tail;
@@ -85,7 +88,7 @@ void CordzHandle::Delete(CordzHandle* handle) {
 
 std::vector<const CordzHandle*> CordzHandle::DiagnosticsGetDeleteQueue() {
   std::vector<const CordzHandle*> handles;
-  MutexLock lock(&global_queue_.mutex);
+  SpinLockHolder lock(&global_queue_.mutex);
   CordzHandle* dq_tail = global_queue_.dq_tail.load(std::memory_order_acquire);
   for (const CordzHandle* p = dq_tail; p; p = p->dq_prev_) {
     handles.push_back(p);
@@ -100,7 +103,7 @@ bool CordzHandle::DiagnosticsHandleIsSafeToInspect(
   if (handle == nullptr) return true;
   if (handle->is_snapshot_) return false;
   bool snapshot_found = false;
-  MutexLock lock(&queue_->mutex);
+  SpinLockHolder lock(&queue_->mutex);
   for (const CordzHandle* p = queue_->dq_tail; p; p = p->dq_prev_) {
     if (p == handle) return !snapshot_found;
     if (p == this) snapshot_found = true;
@@ -117,7 +120,7 @@ CordzHandle::DiagnosticsGetSafeToInspectDeletedHandles() {
     return handles;
   }
 
-  MutexLock lock(&queue_->mutex);
+  SpinLockHolder lock(&queue_->mutex);
   for (const CordzHandle* p = dq_next_; p != nullptr; p = p->dq_next_) {
     if (!p->is_snapshot()) {
       handles.push_back(p);
diff --git a/absl/strings/internal/cordz_handle.h b/absl/strings/internal/cordz_handle.h
index 71563c8b..93076cfd 100644
--- a/absl/strings/internal/cordz_handle.h
+++ b/absl/strings/internal/cordz_handle.h
@@ -20,6 +20,7 @@
 
 #include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/spinlock.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
@@ -70,9 +71,11 @@ class CordzHandle {
   // Global queue data. CordzHandle stores a pointer to the global queue
   // instance to harden against ODR violations.
   struct Queue {
-    constexpr explicit Queue(absl::ConstInitType) : mutex(absl::kConstInit) {}
+    constexpr explicit Queue(absl::ConstInitType)
+        : mutex(absl::kConstInit,
+                absl::base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL) {}
 
-    absl::Mutex mutex;
+    absl::base_internal::SpinLock mutex;
     std::atomic<CordzHandle*> dq_tail ABSL_GUARDED_BY(mutex){nullptr};
 
     // Returns true if this delete queue is empty. This method does not acquire
diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
index d2200680..0461ec46 100644
--- a/absl/strings/internal/cordz_info.cc
+++ b/absl/strings/internal/cordz_info.cc
@@ -15,6 +15,7 @@
 #include "absl/strings/internal/cordz_info.h"
 
 #include "absl/base/config.h"
+#include "absl/base/internal/spinlock.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/strings/internal/cord_internal.h"
 #include "absl/strings/internal/cordz_handle.h"
@@ -27,22 +28,34 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
+using ::absl::base_internal::SpinLockHolder;
+
 constexpr int CordzInfo::kMaxStackDepth;
 
-ABSL_CONST_INIT std::atomic<CordzInfo*> CordzInfo::ci_head_{nullptr};
-ABSL_CONST_INIT absl::Mutex CordzInfo::ci_mutex_(absl::kConstInit);
+ABSL_CONST_INIT CordzInfo::List CordzInfo::global_list_{absl::kConstInit};
 
 CordzInfo* CordzInfo::Head(const CordzSnapshot& snapshot) {
   ABSL_ASSERT(snapshot.is_snapshot());
-  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(ci_head_unsafe()));
-  return ci_head_unsafe();
+
+  // We can do an 'unsafe' load of 'head', as we are guaranteed that the
+  // instance it points to is kept alive by the provided CordzSnapshot, so we
+  // can simply return the current value using an acquire load.
+  // We do enforce in DEBUG builds that the 'head' value is present in the
+  // delete queue: ODR violations may lead to 'snapshot' and 'global_list_'
+  // being in different libraries / modules.
+  CordzInfo* head = global_list_.head.load(std::memory_order_acquire);
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(head));
+  return head;
 }
 
 CordzInfo* CordzInfo::Next(const CordzSnapshot& snapshot) const {
   ABSL_ASSERT(snapshot.is_snapshot());
+
+  // Similar to the 'Head()' function, we do not need a mutex here.
+  CordzInfo* next = ci_next_.load(std::memory_order_acquire);
   ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(this));
-  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(ci_next_unsafe()));
-  return ci_next_unsafe();
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(next));
+  return next;
 }
 
 void CordzInfo::TrackCord(InlineData& cord, MethodIdentifier method) {
@@ -63,14 +76,6 @@ void CordzInfo::TrackCord(InlineData& cord, const InlineData& src,
   cordz_info->Track();
 }
 
-void CordzInfo::UntrackCord(CordzInfo* cordz_info) {
-  assert(cordz_info);
-  if (cordz_info) {
-    cordz_info->Untrack();
-    CordzHandle::Delete(cordz_info);
-  }
-}
-
 CordzInfo::MethodIdentifier CordzInfo::GetParentMethod(const CordzInfo* src) {
   if (src == nullptr) return MethodIdentifier::kUnknown;
   return src->parent_method_ != MethodIdentifier::kUnknown ? src->parent_method_
@@ -110,14 +115,14 @@ CordzInfo::~CordzInfo() {
 }
 
 void CordzInfo::Track() {
-  absl::MutexLock l(&ci_mutex_);
+  SpinLockHolder l(&list_->mutex);
 
-  CordzInfo* const head = ci_head_.load(std::memory_order_acquire);
+  CordzInfo* const head = list_->head.load(std::memory_order_acquire);
   if (head != nullptr) {
     head->ci_prev_.store(this, std::memory_order_release);
   }
   ci_next_.store(head, std::memory_order_release);
-  ci_head_.store(this, std::memory_order_release);
+  list_->head.store(this, std::memory_order_release);
 }
 
 void CordzInfo::Untrack() {
@@ -128,24 +133,28 @@ void CordzInfo::Untrack() {
     rep_ = nullptr;
   }
 
-  absl::MutexLock l(&ci_mutex_);
-
-  CordzInfo* const head = ci_head_.load(std::memory_order_acquire);
-  CordzInfo* const next = ci_next_.load(std::memory_order_acquire);
-  CordzInfo* const prev = ci_prev_.load(std::memory_order_acquire);
-
-  if (next) {
-    ABSL_ASSERT(next->ci_prev_.load(std::memory_order_acquire) == this);
-    next->ci_prev_.store(prev, std::memory_order_release);
-  }
-  if (prev) {
-    ABSL_ASSERT(head != this);
-    ABSL_ASSERT(prev->ci_next_.load(std::memory_order_acquire) == this);
-    prev->ci_next_.store(next, std::memory_order_release);
-  } else {
-    ABSL_ASSERT(head == this);
-    ci_head_.store(next, std::memory_order_release);
+  ODRCheck();
+  {
+    SpinLockHolder l(&list_->mutex);
+
+    CordzInfo* const head = list_->head.load(std::memory_order_acquire);
+    CordzInfo* const next = ci_next_.load(std::memory_order_acquire);
+    CordzInfo* const prev = ci_prev_.load(std::memory_order_acquire);
+
+    if (next) {
+      ABSL_ASSERT(next->ci_prev_.load(std::memory_order_acquire) == this);
+      next->ci_prev_.store(prev, std::memory_order_release);
+    }
+    if (prev) {
+      ABSL_ASSERT(head != this);
+      ABSL_ASSERT(prev->ci_next_.load(std::memory_order_acquire) == this);
+      prev->ci_next_.store(next, std::memory_order_release);
+    } else {
+      ABSL_ASSERT(head == this);
+      list_->head.store(next, std::memory_order_release);
+    }
   }
+  CordzHandle::Delete(this);
 }
 
 void CordzInfo::Lock(MethodIdentifier method)
@@ -163,7 +172,6 @@ void CordzInfo::Unlock() ABSL_UNLOCK_FUNCTION(mutex_) {
   mutex_.Unlock();
   if (!tracked) {
     Untrack();
-    CordzHandle::Delete(this);
   }
 }
 
diff --git a/absl/strings/internal/cordz_info.h b/absl/strings/internal/cordz_info.h
index 4cf57664..f7682cb3 100644
--- a/absl/strings/internal/cordz_info.h
+++ b/absl/strings/internal/cordz_info.h
@@ -20,6 +20,8 @@
 #include <functional>
 
 #include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/spinlock.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/strings/internal/cord_internal.h"
 #include "absl/strings/internal/cordz_functions.h"
@@ -79,17 +81,22 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
   // and before the root cordrep of the sampled cord is unreffed.
   // This function may extend the lifetime of the cordrep in cases where the
   // CordInfo instance is being held by a concurrent collection thread.
-  static void UntrackCord(CordzInfo* cordz_info);
+  void Untrack();
+
+  // Invokes UntrackCord() on `info` if `info` is not null.
+  static void MaybeUntrackCord(CordzInfo* info);
 
   CordzInfo() = delete;
   CordzInfo(const CordzInfo&) = delete;
   CordzInfo& operator=(const CordzInfo&) = delete;
 
   // Retrieves the oldest existing CordzInfo.
-  static CordzInfo* Head(const CordzSnapshot& snapshot);
+  static CordzInfo* Head(const CordzSnapshot& snapshot)
+      ABSL_NO_THREAD_SAFETY_ANALYSIS;
 
   // Retrieves the next oldest existing CordzInfo older than 'this' instance.
-  CordzInfo* Next(const CordzSnapshot& snapshot) const;
+  CordzInfo* Next(const CordzSnapshot& snapshot) const
+      ABSL_NO_THREAD_SAFETY_ANALYSIS;
 
   // Locks this instance for the update identified by `method`.
   // Increases the count for `method` in `update_tracker`.
@@ -141,6 +148,17 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
   }
 
  private:
+  // Global cordz info list. CordzInfo stores a pointer to the global list
+  // instance to harden against ODR violations.
+  struct List {
+    constexpr explicit List(absl::ConstInitType)
+        : mutex(absl::kConstInit,
+                absl::base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL) {}
+
+    absl::base_internal::SpinLock mutex;
+    std::atomic<CordzInfo*> head ABSL_GUARDED_BY(mutex){nullptr};
+  };
+
   static constexpr int kMaxStackDepth = 64;
 
   explicit CordzInfo(CordRep* rep, const CordzInfo* src,
@@ -148,7 +166,6 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
   ~CordzInfo() override;
 
   void Track();
-  void Untrack();
 
   // Returns the parent method from `src`, which is either `parent_method_` or
   // `method_` depending on `parent_method_` being kUnknown.
@@ -161,23 +178,20 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
   // Returns 0 if `src` is null.
   static int FillParentStack(const CordzInfo* src, void** stack);
 
-  // 'Unsafe' head/next/prev accessors not requiring the lock being held.
-  // These are used exclusively for iterations (Head / Next) where we enforce
-  // a token being held, so reading an 'old' / deleted pointer is fine.
-  static CordzInfo* ci_head_unsafe() ABSL_NO_THREAD_SAFETY_ANALYSIS {
-    return ci_head_.load(std::memory_order_acquire);
-  }
-  CordzInfo* ci_next_unsafe() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
-    return ci_next_.load(std::memory_order_acquire);
-  }
-  CordzInfo* ci_prev_unsafe() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
-    return ci_prev_.load(std::memory_order_acquire);
+  void ODRCheck() const {
+#ifndef NDEBUG
+    ABSL_RAW_CHECK(list_ == &global_list_, "ODR violation in Cord");
+#endif
   }
 
-  static absl::Mutex ci_mutex_;
-  static std::atomic<CordzInfo*> ci_head_ ABSL_GUARDED_BY(ci_mutex_);
-  std::atomic<CordzInfo*> ci_prev_ ABSL_GUARDED_BY(ci_mutex_){nullptr};
-  std::atomic<CordzInfo*> ci_next_ ABSL_GUARDED_BY(ci_mutex_){nullptr};
+  ABSL_CONST_INIT static List global_list_;
+  List* const list_ = &global_list_;
+
+  // ci_prev_ and ci_next_ require the global list mutex to be held.
+  // Unfortunately we can't use thread annotations such that the thread safety
+  // analysis understands that list_ and global_list_ are one and the same.
+  std::atomic<CordzInfo*> ci_prev_{nullptr};
+  std::atomic<CordzInfo*> ci_next_{nullptr};
 
   mutable absl::Mutex mutex_;
   CordRep* rep_ ABSL_GUARDED_BY(mutex_);
@@ -209,6 +223,13 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeTrackCord(
   }
 }
 
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeUntrackCord(
+    CordzInfo* info) {
+  if (ABSL_PREDICT_FALSE(info)) {
+    info->Untrack();
+  }
+}
+
 inline void CordzInfo::AssertHeld() ABSL_ASSERT_EXCLUSIVE_LOCK(mutex_) {
 #ifndef NDEBUG
   mutex_.AssertHeld();
diff --git a/absl/strings/internal/cordz_info_test.cc b/absl/strings/internal/cordz_info_test.cc
index 76aa20b0..5eaf4b9c 100644
--- a/absl/strings/internal/cordz_info_test.cc
+++ b/absl/strings/internal/cordz_info_test.cc
@@ -70,7 +70,7 @@ TEST(CordzInfoTest, TrackCord) {
   EXPECT_FALSE(info->is_snapshot());
   EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(info));
   EXPECT_THAT(info->GetCordRepForTesting(), Eq(data.rep.rep));
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 TEST(CordzInfoTest, UntrackCord) {
@@ -79,7 +79,7 @@ TEST(CordzInfoTest, UntrackCord) {
   CordzInfo* info = data.data.cordz_info();
 
   CordzSnapshot snapshot;
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
   EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(nullptr));
   EXPECT_THAT(info->GetCordRepForTesting(), Eq(nullptr));
   EXPECT_THAT(DeleteQueue(), ElementsAre(info, &snapshot));
@@ -96,7 +96,7 @@ TEST(CordzInfoTest, SetCordRep) {
   info->Unlock();
   EXPECT_THAT(info->GetCordRepForTesting(), Eq(rep.rep));
 
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 TEST(CordzInfoTest, SetCordRepNullUntracksCordOnUnlock) {
@@ -121,7 +121,7 @@ TEST(CordzInfoTest, SetCordRepRequiresMutex) {
   CordzInfo* info = data.data.cordz_info();
   TestCordRep rep;
   EXPECT_DEBUG_DEATH(info->SetCordRep(rep.rep), ".*");
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 #endif  // GTEST_HAS_DEATH_TEST
@@ -143,11 +143,11 @@ TEST(CordzInfoTest, TrackUntrackHeadFirstV2) {
   EXPECT_THAT(info2->Next(snapshot), Eq(info1));
   EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
 
-  CordzInfo::UntrackCord(info2);
+  info2->Untrack();
   ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info1));
   EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
 
-  CordzInfo::UntrackCord(info1);
+  info1->Untrack();
   ASSERT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
 }
 
@@ -168,11 +168,11 @@ TEST(CordzInfoTest, TrackUntrackTailFirstV2) {
   EXPECT_THAT(info2->Next(snapshot), Eq(info1));
   EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
 
-  CordzInfo::UntrackCord(info1);
+  info1->Untrack();
   ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info2));
   EXPECT_THAT(info2->Next(snapshot), Eq(nullptr));
 
-  CordzInfo::UntrackCord(info2);
+  info2->Untrack();
   ASSERT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
 }
 
@@ -208,7 +208,7 @@ TEST(CordzInfoTest, StackV2) {
   // got_stack.
   EXPECT_THAT(got_stack, HasSubstr(expected_stack));
 
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 // Local helper functions to get different stacks for child and parent.
@@ -231,7 +231,7 @@ TEST(CordzInfoTest, GetStatistics) {
   EXPECT_THAT(statistics.parent_method, Eq(kUnknownMethod));
   EXPECT_THAT(statistics.update_tracker.Value(kTrackCordMethod), Eq(1));
 
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 TEST(CordzInfoTest, LockCountsMethod) {
@@ -246,7 +246,7 @@ TEST(CordzInfoTest, LockCountsMethod) {
   CordzStatistics statistics = info->GetCordzStatistics();
   EXPECT_THAT(statistics.update_tracker.Value(kUpdateMethod), Eq(2));
 
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 TEST(CordzInfoTest, FromParent) {
@@ -265,8 +265,8 @@ TEST(CordzInfoTest, FromParent) {
   EXPECT_THAT(statistics.parent_method, Eq(kTrackCordMethod));
   EXPECT_THAT(statistics.update_tracker.Value(kChildMethod), Eq(1));
 
-  CordzInfo::UntrackCord(info_parent);
-  CordzInfo::UntrackCord(info_child);
+  info_parent->Untrack();
+  info_child->Untrack();
 }
 
 TEST(CordzInfoTest, FromParentInlined) {
@@ -279,7 +279,7 @@ TEST(CordzInfoTest, FromParentInlined) {
   EXPECT_THAT(statistics.method, Eq(kChildMethod));
   EXPECT_THAT(statistics.parent_method, Eq(kUnknownMethod));
   EXPECT_THAT(statistics.update_tracker.Value(kChildMethod), Eq(1));
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 TEST(CordzInfoTest, RecordMetrics) {
@@ -293,7 +293,7 @@ TEST(CordzInfoTest, RecordMetrics) {
   CordzStatistics actual = info->GetCordzStatistics();
   EXPECT_EQ(actual.size, expected.size);
 
-  CordzInfo::UntrackCord(info);
+  info->Untrack();
 }
 
 }  // namespace
diff --git a/absl/strings/internal/cordz_sample_token_test.cc b/absl/strings/internal/cordz_sample_token_test.cc
index 80f41ca1..9f54301d 100644
--- a/absl/strings/internal/cordz_sample_token_test.cc
+++ b/absl/strings/internal/cordz_sample_token_test.cc
@@ -96,9 +96,9 @@ TEST(CordzSampleTokenTest, Iterator) {
 
   EXPECT_THAT(found, ElementsAre(info3, info2, info1));
 
-  CordzInfo::UntrackCord(info1);
-  CordzInfo::UntrackCord(info2);
-  CordzInfo::UntrackCord(info3);
+  info1->Untrack();
+  info2->Untrack();
+  info3->Untrack();
 }
 
 TEST(CordzSampleTokenTest, IteratorEquality) {
@@ -135,9 +135,9 @@ TEST(CordzSampleTokenTest, IteratorEquality) {
   // Both lhs and rhs are done, so they are on nullptr.
   EXPECT_THAT(lhs, Eq(rhs));
 
-  CordzInfo::UntrackCord(info1);
-  CordzInfo::UntrackCord(info2);
-  CordzInfo::UntrackCord(info3);
+  info1->Untrack();
+  info2->Untrack();
+  info3->Untrack();
 }
 
 TEST(CordzSampleTokenTest, MultiThreaded) {
@@ -166,7 +166,7 @@ TEST(CordzSampleTokenTest, MultiThreaded) {
           // Track/untrack.
           if (cord.data.is_profiled()) {
             // 1) Untrack
-            CordzInfo::UntrackCord(cord.data.cordz_info());
+            cord.data.cordz_info()->Untrack();
             cord.data.clear_cordz_info();;
           } else {
             // 2) Track
@@ -192,9 +192,7 @@ TEST(CordzSampleTokenTest, MultiThreaded) {
         }
       }
       for (TestCordData& cord : cords) {
-        if (cord.data.is_profiled()) {
-          CordzInfo::UntrackCord(cord.data.cordz_info());
-        }
+        CordzInfo::MaybeUntrackCord(cord.data.cordz_info());
       }
     });
   }
diff --git a/absl/strings/internal/cordz_update_scope.h b/absl/strings/internal/cordz_update_scope.h
index 018359a8..57ba75de 100644
--- a/absl/strings/internal/cordz_update_scope.h
+++ b/absl/strings/internal/cordz_update_scope.h
@@ -40,6 +40,12 @@ class ABSL_SCOPED_LOCKABLE CordzUpdateScope {
     }
   }
 
+  // CordzUpdateScope can not be copied or assigned to.
+  CordzUpdateScope(CordzUpdateScope&& rhs) = delete;
+  CordzUpdateScope(const CordzUpdateScope&) = delete;
+  CordzUpdateScope& operator=(CordzUpdateScope&& rhs) = delete;
+  CordzUpdateScope& operator=(const CordzUpdateScope&) = delete;
+
   ~CordzUpdateScope() ABSL_UNLOCK_FUNCTION() {
     if (ABSL_PREDICT_FALSE(info_)) {
       info_->Unlock();
@@ -55,7 +61,7 @@ class ABSL_SCOPED_LOCKABLE CordzUpdateScope {
   CordzInfo* info() const { return info_; }
 
  private:
-  CordzInfo* const info_;
+  CordzInfo* info_;
 };
 
 }  // namespace cord_internal