1 files changed, 17 insertions, 5 deletions
diff --git a/abseil-cpp/absl/base/internal/spinlock.cc b/abseil-cpp/absl/base/internal/spinlock.cc
index a7d44f3..381b913 100644
--- a/abseil-cpp/absl/base/internal/spinlock.cc
+++ b/abseil-cpp/absl/base/internal/spinlock.cc
@@ -19,6 +19,7 @@
 #include <limits>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/internal/cycleclock.h"
 #include "absl/base/internal/spinlock_wait.h"
@@ -66,12 +67,14 @@ void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
   submit_profile_data.Store(fn);
 }
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 // Static member variable definitions.
 constexpr uint32_t SpinLock::kSpinLockHeld;
 constexpr uint32_t SpinLock::kSpinLockCooperative;
 constexpr uint32_t SpinLock::kSpinLockDisabledScheduling;
 constexpr uint32_t SpinLock::kSpinLockSleeper;
 constexpr uint32_t SpinLock::kWaitTimeMask;
+#endif
 
 // Uncommon constructors.
 SpinLock::SpinLock(base_internal::SchedulingMode mode)
@@ -125,8 +128,9 @@ 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 +144,14 @@ 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;
       }
     }
 
@@ -166,7 +178,7 @@ void SpinLock::SlowUnlock(uint32_t lock_value) {
   // reserve a unitary wait time to represent that a waiter exists without our
   // own acquisition having been contended.
   if ((lock_value & kWaitTimeMask) != kSpinLockSleeper) {
-    const uint64_t wait_cycles = DecodeWaitCycles(lock_value);
+    const int64_t wait_cycles = DecodeWaitCycles(lock_value);
     ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
     submit_profile_data(this, wait_cycles);
     ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
@@ -208,9 +220,9 @@ uint32_t SpinLock::EncodeWaitCycles(int64_t wait_start_time,
   return clamped;
 }
 
-uint64_t SpinLock::DecodeWaitCycles(uint32_t lock_value) {
+int64_t SpinLock::DecodeWaitCycles(uint32_t lock_value) {
   // Cast to uint32_t first to ensure bits [63:32] are cleared.
-  const uint64_t scaled_wait_time =
+  const int64_t scaled_wait_time =
       static_cast<uint32_t>(lock_value & kWaitTimeMask);
   return scaled_wait_time << (kProfileTimestampShift - kLockwordReservedShift);
 }