diff options
Diffstat (limited to 'absl/synchronization/mutex.cc')
| -rw-r--r-- | absl/synchronization/mutex.cc | 68 |
1 files changed, 33 insertions, 35 deletions
diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc index 82b631af..d2468ce5 100644 --- a/absl/synchronization/mutex.cc +++ b/absl/synchronization/mutex.cc @@ -761,11 +761,13 @@ void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode) { synch_deadlock_detection.store(mode, std::memory_order_release); } -// Return true iff threads x and y are waiting on the same condition for the -// same type of lock. Requires that x and y be waiting on the same Mutex -// queue. -static bool MuSameCondition(PerThreadSynch *x, PerThreadSynch *y) { - return x->waitp->how == y->waitp->how && +// Return true iff threads x and y are part of the same equivalence +// class of waiters. An equivalence class is defined as the set of +// waiters with the same condition, type of lock, and thread priority. +// +// Requires that x and y be waiting on the same Mutex queue. +static bool MuEquivalentWaiter(PerThreadSynch *x, PerThreadSynch *y) { + return x->waitp->how == y->waitp->how && x->priority == y->priority && Condition::GuaranteedEqual(x->waitp->cond, y->waitp->cond); } @@ -784,18 +786,19 @@ static inline PerThreadSynch *GetPerThreadSynch(intptr_t v) { // - invalid (iff x is not in a Mutex wait queue), // - null, or // - a pointer to a distinct thread waiting later in the same Mutex queue -// such that all threads in [x, x->skip] have the same condition and -// lock type (MuSameCondition() is true for all pairs in [x, x->skip]). +// such that all threads in [x, x->skip] have the same condition, priority +// and lock type (MuEquivalentWaiter() is true for all pairs in [x, +// x->skip]). // In addition, if x->skip is valid, (x->may_skip || x->skip == null) // -// By the spec of MuSameCondition(), it is not necessary when removing the +// By the spec of MuEquivalentWaiter(), it is not necessary when removing the // first runnable thread y from the front a Mutex queue to adjust the skip // field of another thread x because if x->skip==y, x->skip must (have) become // invalid before y is removed. The function TryRemove can remove a specified // thread from an arbitrary position in the queue whether runnable or not, so // it fixes up skip fields that would otherwise be left dangling. // The statement -// if (x->may_skip && MuSameCondition(x, x->next)) { x->skip = x->next; } +// if (x->may_skip && MuEquivalentWaiter(x, x->next)) { x->skip = x->next; } // maintains the invariant provided x is not the last waiter in a Mutex queue // The statement // if (x->skip != null) { x->skip = x->skip->skip; } @@ -929,24 +932,17 @@ static PerThreadSynch *Enqueue(PerThreadSynch *head, if (s->priority > head->priority) { // s's priority is above head's // try to put s in priority-fifo order, or failing that at the front. if (!head->maybe_unlocking) { - // No unlocker can be scanning the queue, so we can insert between - // skip-chains, and within a skip-chain if it has the same condition as - // s. We insert in priority-fifo order, examining the end of every - // skip-chain, plus every element with the same condition as s. + // No unlocker can be scanning the queue, so we can insert into the + // middle of the queue. + // + // Within a skip chain, all waiters have the same priority, so we can + // skip forward through the chains until we find one with a lower + // priority than the waiter to be enqueued. PerThreadSynch *advance_to = head; // next value of enqueue_after - PerThreadSynch *cur; // successor of enqueue_after do { enqueue_after = advance_to; - cur = enqueue_after->next; // this advance ensures progress - advance_to = Skip(cur); // normally, advance to end of skip chain - // (side-effect: optimizes skip chain) - if (advance_to != cur && s->priority > advance_to->priority && - MuSameCondition(s, cur)) { - // but this skip chain is not a singleton, s has higher priority - // than its tail and has the same condition as the chain, - // so we can insert within the skip-chain - advance_to = cur; // advance by just one - } + // (side-effect: optimizes skip chain) + advance_to = Skip(enqueue_after->next); } while (s->priority <= advance_to->priority); // termination guaranteed because s->priority > head->priority // and head is the end of a skip chain @@ -965,21 +961,21 @@ static PerThreadSynch *Enqueue(PerThreadSynch *head, // enqueue_after can be: head, Skip(...), or cur. // The first two imply enqueue_after->skip == nullptr, and - // the last is used only if MuSameCondition(s, cur). + // the last is used only if MuEquivalentWaiter(s, cur). // We require this because clearing enqueue_after->skip // is impossible; enqueue_after's predecessors might also // incorrectly skip over s if we were to allow other // insertion points. - ABSL_RAW_CHECK( - enqueue_after->skip == nullptr || MuSameCondition(enqueue_after, s), - "Mutex Enqueue failure"); + ABSL_RAW_CHECK(enqueue_after->skip == nullptr || + MuEquivalentWaiter(enqueue_after, s), + "Mutex Enqueue failure"); if (enqueue_after != head && enqueue_after->may_skip && - MuSameCondition(enqueue_after, enqueue_after->next)) { + MuEquivalentWaiter(enqueue_after, enqueue_after->next)) { // enqueue_after can skip to its new successor, s enqueue_after->skip = enqueue_after->next; } - if (MuSameCondition(s, s->next)) { // s->may_skip is known to be true + if (MuEquivalentWaiter(s, s->next)) { // s->may_skip is known to be true s->skip = s->next; // s may skip to its successor } } else { // enqueue not done any other way, so @@ -989,7 +985,7 @@ static PerThreadSynch *Enqueue(PerThreadSynch *head, head->next = s; s->readers = head->readers; // reader count is from previous head s->maybe_unlocking = head->maybe_unlocking; // same for unlock hint - if (head->may_skip && MuSameCondition(head, s)) { + if (head->may_skip && MuEquivalentWaiter(head, s)) { // head now has successor; may skip head->skip = s; } @@ -1009,7 +1005,7 @@ static PerThreadSynch *Dequeue(PerThreadSynch *head, PerThreadSynch *pw) { pw->next = w->next; // snip w out of list if (head == w) { // we removed the head head = (pw == w) ? nullptr : pw; // either emptied list, or pw is new head - } else if (pw != head && MuSameCondition(pw, pw->next)) { + } else if (pw != head && MuEquivalentWaiter(pw, pw->next)) { // pw can skip to its new successor if (pw->next->skip != nullptr) { // either skip to its successors skip target @@ -1079,11 +1075,13 @@ void Mutex::TryRemove(PerThreadSynch *s) { PerThreadSynch *w; if ((w = pw->next) != s) { // search for thread, do { // processing at least one element - if (!MuSameCondition(s, w)) { // seeking different condition + // If the current element isn't equivalent to the waiter to be + // removed, we can skip the entire chain. + if (!MuEquivalentWaiter(s, w)) { pw = Skip(w); // so skip all that won't match // we don't have to worry about dangling skip fields // in the threads we skipped; none can point to s - // because their condition differs from s + // because they are in a different equivalence class. } else { // seeking same condition FixSkip(w, s); // fix up any skip pointer from w to s pw = w; @@ -2148,7 +2146,7 @@ ABSL_ATTRIBUTE_NOINLINE void Mutex::UnlockSlow(SynchWaitParams *waitp) { !old_h->may_skip) { // we used old_h as a terminator old_h->may_skip = true; // allow old_h to skip once more ABSL_RAW_CHECK(old_h->skip == nullptr, "illegal skip from head"); - if (h != old_h && MuSameCondition(old_h, old_h->next)) { + if (h != old_h && MuEquivalentWaiter(old_h, old_h->next)) { old_h->skip = old_h->next; // old_h not head & can skip to successor } } |