diff options
Diffstat (limited to 'absl/base/internal/spinlock.h')
| -rw-r--r-- | absl/base/internal/spinlock.h | 19 |
1 files changed, 14 insertions, 5 deletions
diff --git a/absl/base/internal/spinlock.h b/absl/base/internal/spinlock.h index e6ac9e64..c73b5e09 100644 --- a/absl/base/internal/spinlock.h +++ b/absl/base/internal/spinlock.h @@ -15,11 +15,8 @@ // // Most users requiring mutual exclusion should use Mutex. -// SpinLock is provided for use in three situations: +// SpinLock is provided for use in two situations: // - for use in code that Mutex itself depends on -// - to get a faster fast-path release under low contention (without an -// atomic read-modify-write) In return, SpinLock has worse behaviour under -// contention, which is why Mutex is preferred in most situations. // - for async signal safety (see below) // SpinLock is async signal safe. If a spinlock is used within a signal @@ -140,8 +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; |