2 files changed, 14 insertions, 11 deletions
diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 3af4cda9..376ea794 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -109,7 +109,7 @@ static inline bool EvalConditionAnnotated(const Condition *cond, Mutex *mu,
                                           bool locking, bool trylock,
                                           bool read_lock);
 
-void RegisterMutexProfiler(void (*fn)(int64_t wait_timestamp)) {
+void RegisterMutexProfiler(void (*fn)(int64_t wait_cycles)) {
   submit_profile_data.Store(fn);
 }
 
@@ -2315,16 +2315,18 @@ ABSL_ATTRIBUTE_NOINLINE void Mutex::UnlockSlow(SynchWaitParams *waitp) {
   }                            // end of for(;;)-loop
 
   if (wake_list != kPerThreadSynchNull) {
-    int64_t enqueue_timestamp = wake_list->waitp->contention_start_cycles;
-    bool cond_waiter = wake_list->cond_waiter;
+    int64_t wait_cycles = 0;
+    int64_t now = base_internal::CycleClock::Now();
     do {
+      // Sample lock contention events only if the waiter was trying to acquire
+      // the lock, not waiting on a condition variable or Condition.
+      if (!wake_list->cond_waiter) {
+        wait_cycles += (now - wake_list->waitp->contention_start_cycles);
+        wake_list->waitp->contention_start_cycles = now;
+      }
       wake_list = Wakeup(wake_list);              // wake waiters
     } while (wake_list != kPerThreadSynchNull);
-    if (!cond_waiter) {
-      // Sample lock contention events only if the (first) waiter was trying to
-      // acquire the lock, not waiting on a condition variable or Condition.
-      int64_t wait_cycles =
-          base_internal::CycleClock::Now() - enqueue_timestamp;
+    if (wait_cycles > 0) {
       mutex_tracer("slow release", this, wait_cycles);
       ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
       submit_profile_data(wait_cycles);
diff --git a/absl/synchronization/mutex.h b/absl/synchronization/mutex.h
index 38338f24..9a3e438f 100644
--- a/absl/synchronization/mutex.h
+++ b/absl/synchronization/mutex.h
@@ -984,14 +984,15 @@ inline Condition::Condition(const T *object,
 // Register a hook for profiling support.
 //
 // The function pointer registered here will be called whenever a mutex is
-// contended.  The callback is given the absl/base/cycleclock.h timestamp when
-// waiting began.
+// contended.  The callback is given the cycles for which waiting happened (as
+// measured by //absl/base/internal/cycleclock.h, and which may not
+// be real "cycle" counts.)
 //
 // Calls to this function do not race or block, but there is no ordering
 // guaranteed between calls to this function and call to the provided hook.
 // In particular, the previously registered hook may still be called for some
 // time after this function returns.
-void RegisterMutexProfiler(void (*fn)(int64_t wait_timestamp));
+void RegisterMutexProfiler(void (*fn)(int64_t wait_cycles));
 
 // Register a hook for Mutex tracing.
 //