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Export of internal Abseil changes

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

Internal change

PiperOrigin-RevId: 352140672
GitOrigin-RevId: 874f906d2b90d4c74b0edeac52811efc10323422
Change-Id: I151299caff75d404309f059dd0e5971f5a71655d
This commit is contained in:
Abseil Team
2021-01-15 22:34:04 -08:00
committed by vslashg
parent ff361eb3aa
commit b2dcbba183
2 changed files with 24 additions and 3 deletions
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13
absl/base/internal/spinlock.cc
View File

@@ -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;
}
}

14
absl/base/internal/spinlock.h
View File

@@ -137,8 +137,20 @@ class ABSL_LOCKABLE SpinLock {
//
// bit[0] encodes whether a lock is being held.
// bit[1] encodes whether a lock uses cooperative scheduling.
// bit[2] encodes whether a lock disables scheduling.
// bit[2] encodes whether the current lock holder disabled scheduling when
// acquiring the lock. Only set when kSpinLockHeld is also set.
// bit[3:31] encodes time a lock spent on waiting as a 29-bit unsigned int.
// This is set by the lock holder to indicate how long it waited on
// the lock before eventually acquiring it. The number of cycles is
// encoded as a 29-bit unsigned int, or in the case that the current
// holder did not wait but another waiter is queued, the LSB
// (kSpinLockSleeper) is set. The implementation does not explicitly
// track the number of queued waiters beyond this. It must always be
// assumed that waiters may exist if the current holder was required to
// queue.
//
// Invariant: if the lock is not held, the value is either 0 or
// kSpinLockCooperative.
static constexpr uint32_t kSpinLockHeld = 1;
static constexpr uint32_t kSpinLockCooperative = 2;
static constexpr uint32_t kSpinLockDisabledScheduling = 4;