Export of internal Abseil changes

--
874f906d2b90d4c74b0edeac52811efc10323422 by Todd Lipcon <tlipcon@google.com>:

Internal change

PiperOrigin-RevId: 352140672
GitOrigin-RevId: 874f906d2b90d4c74b0edeac52811efc10323422
Change-Id: I151299caff75d404309f059dd0e5971f5a71655d

1 files changed, 11 insertions, 2 deletions

diff --git a/absl/base/internal/spinlock.cc b/absl/base/internal/spinlock.cc
index a7d44f3e..35c0696a 100644
--- a/absl/base/internal/spinlock.cc
+++ b/absl/base/internal/spinlock.cc
@@ -125,8 +125,9 @@ void SpinLock::SlowLock() {
     // it as having a sleeper.
     if ((lock_value & kWaitTimeMask) == 0) {
       // Here, just "mark" that the thread is going to sleep.  Don't store the
-      // lock wait time in the lock as that will cause the current lock
-      // owner to think it experienced contention.
+      // lock wait time in the lock -- the lock word stores the amount of time
+      // that the current holder waited before acquiring the lock, not the wait
+      // time of any thread currently waiting to acquire it.
       if (lockword_.compare_exchange_strong(
               lock_value, lock_value | kSpinLockSleeper,
               std::memory_order_relaxed, std::memory_order_relaxed)) {
@@ -140,6 +141,14 @@ void SpinLock::SlowLock() {
         // this thread obtains the lock.
         lock_value = TryLockInternal(lock_value, wait_cycles);
         continue;   // Skip the delay at the end of the loop.
+      } else if ((lock_value & kWaitTimeMask) == 0) {
+        // The lock is still held, without a waiter being marked, but something
+        // else about the lock word changed, causing our CAS to fail. For
+        // example, a new lock holder may have acquired the lock with
+        // kSpinLockDisabledScheduling set, whereas the previous holder had not
+        // set that flag. In this case, attempt again to mark ourselves as a
+        // waiter.
+        continue;
       }
     }