rw: whack avoidable re-reads in try_upgrade
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parent
bda8aa770a
commit
04457342a3
Notes:
svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=332394
@ -1204,7 +1204,7 @@ __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF)
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int
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__rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
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{
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uintptr_t v, x, tid;
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uintptr_t v, setv, tid;
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struct turnstile *ts;
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int success;
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@ -1224,12 +1224,12 @@ __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
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*/
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tid = (uintptr_t)curthread;
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success = 0;
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v = RW_READ_VALUE(rw);
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for (;;) {
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v = rw->rw_lock;
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if (RW_READERS(v) > 1)
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break;
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if (!(v & RW_LOCK_WAITERS)) {
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success = atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid);
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success = atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid);
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if (!success)
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continue;
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break;
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@ -1239,7 +1239,8 @@ __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
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* Ok, we think we have waiters, so lock the turnstile.
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*/
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ts = turnstile_trywait(&rw->lock_object);
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v = rw->rw_lock;
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v = RW_READ_VALUE(rw);
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retry_ts:
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if (RW_READERS(v) > 1) {
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turnstile_cancel(ts);
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break;
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@ -1250,16 +1251,16 @@ __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
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* If we obtain the lock with the flags set, then claim
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* ownership of the turnstile.
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*/
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x = rw->rw_lock & RW_LOCK_WAITERS;
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success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
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setv = tid | (v & RW_LOCK_WAITERS);
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success = atomic_fcmpset_ptr(&rw->rw_lock, &v, setv);
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if (success) {
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if (x)
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if (v & RW_LOCK_WAITERS)
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turnstile_claim(ts);
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else
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turnstile_cancel(ts);
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break;
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}
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turnstile_cancel(ts);
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goto retry_ts;
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}
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LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
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if (success) {
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