Export of internal Abseil changes

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5bd06440e700fefd6eadd577d7d69c51f15c63e0 by Abseil Team <absl-team@google.com>:

Add if-guard to futex.h so that it doesn't fail to parse for unsupported platforms

PiperOrigin-RevId: 336880375

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8b3d3bb4ad123fc9f648f0e397b2eddd88dc0c02 by Derek Mauro <dmauro@google.com>:

Fix race in AddressIsReadable file descriptors using stronger memory ordering

PiperOrigin-RevId: 336874423

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1d8bf23747009cca29129b80c2793bc91443dd55 by Derek Mauro <dmauro@google.com>:

Avoid -Wundef warnings on ABSL_HAVE_THREAD_LOCAL

PiperOrigin-RevId: 336792406

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562a480f029c600c1d3b1428da6a9b09e8952a74 by Derek Mauro <dmauro@google.com>:

Fix preprocessor condition for symbols __tsan_mutex_read_lock and
__tsan_mutex_try_lock

PiperOrigin-RevId: 336732571
GitOrigin-RevId: 5bd06440e700fefd6eadd577d7d69c51f15c63e0
Change-Id: Id9bb331baec74b9d80c7b228959a7739bc30e694

1 files changed, 5 insertions, 4 deletions

diff --git a/absl/debugging/internal/address_is_readable.cc b/absl/debugging/internal/address_is_readable.cc
index 65376063..329c285f 100644
--- a/absl/debugging/internal/address_is_readable.cc
+++ b/absl/debugging/internal/address_is_readable.cc
@@ -68,6 +68,7 @@ static void Unpack(uint64_t x, int *pid, int *read_fd, int *write_fd) {
 // unimplemented.
 // This is a namespace-scoped variable for correct zero-initialization.
 static std::atomic<uint64_t> pid_and_fds;  // initially 0, an invalid pid.
+
 bool AddressIsReadable(const void *addr) {
   absl::base_internal::ErrnoSaver errno_saver;
   // We test whether a byte is readable by using write().  Normally, this would
@@ -86,7 +87,7 @@ bool AddressIsReadable(const void *addr) {
     int pid;
     int read_fd;
     int write_fd;
-    uint64_t local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
+    uint64_t local_pid_and_fds = pid_and_fds.load(std::memory_order_acquire);
     Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
     while (current_pid != pid) {
       int p[2];
@@ -98,13 +99,13 @@ bool AddressIsReadable(const void *addr) {
       fcntl(p[1], F_SETFD, FD_CLOEXEC);
       uint64_t new_pid_and_fds = Pack(current_pid, p[0], p[1]);
       if (pid_and_fds.compare_exchange_strong(
-              local_pid_and_fds, new_pid_and_fds, std::memory_order_relaxed,
+              local_pid_and_fds, new_pid_and_fds, std::memory_order_release,
               std::memory_order_relaxed)) {
         local_pid_and_fds = new_pid_and_fds;  // fds exposed to other threads
       } else {  // fds not exposed to other threads; we can close them.
         close(p[0]);
         close(p[1]);
-        local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
+        local_pid_and_fds = pid_and_fds.load(std::memory_order_acquire);
       }
       Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
     }
@@ -124,7 +125,7 @@ bool AddressIsReadable(const void *addr) {
       // If pid_and_fds contains the problematic file descriptors we just used,
       // this call will forget them, and the loop will try again.
       pid_and_fds.compare_exchange_strong(local_pid_and_fds, 0,
-                                          std::memory_order_relaxed,
+                                          std::memory_order_release,
                                           std::memory_order_relaxed);
     }
   } while (errno == EBADF);