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rw: whack avoidable re-reads in try_upgrade

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This commit is contained in:
Mateusz Guzik 2018-04-10 22:32:31 +00:00
parent bda8aa770a
commit 04457342a3
1 changed files with 9 additions and 8 deletions
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17
sys/kern/kern_rwlock.c
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@ -1204,7 +1204,7 @@ __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF)
int
__rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
{
uintptr_t v, x, tid;
uintptr_t v, setv, tid;
struct turnstile *ts;
int success;
@ -1224,12 +1224,12 @@ __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
*/
tid = (uintptr_t)curthread;
success = 0;
v = RW_READ_VALUE(rw);
for (;;) {
v = rw->rw_lock;
if (RW_READERS(v) > 1)
break;
if (!(v & RW_LOCK_WAITERS)) {
success = atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid);
success = atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid);
if (!success)
continue;
break;
@ -1239,7 +1239,8 @@ __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
* Ok, we think we have waiters, so lock the turnstile.
*/
ts = turnstile_trywait(&rw->lock_object);
v = rw->rw_lock;
v = RW_READ_VALUE(rw);
retry_ts:
if (RW_READERS(v) > 1) {
turnstile_cancel(ts);
break;
@ -1250,16 +1251,16 @@ __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
* If we obtain the lock with the flags set, then claim
* ownership of the turnstile.
*/
x = rw->rw_lock & RW_LOCK_WAITERS;
success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
setv = tid | (v & RW_LOCK_WAITERS);
success = atomic_fcmpset_ptr(&rw->rw_lock, &v, setv);
if (success) {
if (x)
if (v & RW_LOCK_WAITERS)
turnstile_claim(ts);
else
turnstile_cancel(ts);
break;
}
turnstile_cancel(ts);
goto retry_ts;
}
LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
if (success) {