From 93118b62f9b7a180f15b054b5bf0889f2ef854b0 Mon Sep 17 00:00:00 2001 From: Mateusz Guzik Date: Sat, 25 Nov 2017 20:10:33 +0000 Subject: [PATCH] locks: retry turnstile/sleepq loops on failed cmpset In order to go to sleep threads set waiter flags, but that can spuriously fail e.g. when a new reader arrives. Instead of unlocking everything and looping back, re-evaluate the new state while still holding the lock necessary to go to sleep. --- sys/kern/kern_rwlock.c | 39 ++++++++++++++++----------------------- sys/kern/kern_sx.c | 34 +++++++++++++--------------------- 2 files changed, 29 insertions(+), 44 deletions(-) diff --git a/sys/kern/kern_rwlock.c b/sys/kern/kern_rwlock.c index c0bc0c7510b2..3c2cab87b706 100644 --- a/sys/kern/kern_rwlock.c +++ b/sys/kern/kern_rwlock.c @@ -526,6 +526,7 @@ __rw_rlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v * recheck its state and restart the loop if needed. */ v = RW_READ_VALUE(rw); +retry_ts: if (__rw_can_read(td, v, false)) { turnstile_cancel(ts); continue; @@ -561,12 +562,9 @@ __rw_rlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v * lock and restart the loop. */ if (!(v & RW_LOCK_READ_WAITERS)) { - if (!atomic_cmpset_ptr(&rw->rw_lock, v, - v | RW_LOCK_READ_WAITERS)) { - turnstile_cancel(ts); - v = RW_READ_VALUE(rw); - continue; - } + if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, + v | RW_LOCK_READ_WAITERS)) + goto retry_ts; if (LOCK_LOG_TEST(&rw->lock_object, 0)) CTR2(KTR_LOCK, "%s: %p set read waiters flag", __func__, rw); @@ -757,7 +755,9 @@ __rw_runlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v * last reader, so grab the turnstile lock. */ turnstile_chain_lock(&rw->lock_object); - v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); + v = RW_READ_VALUE(rw); +retry_ts: + v &= (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); MPASS(v & RW_LOCK_WAITERS); /* @@ -782,12 +782,9 @@ __rw_runlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v x |= (v & RW_LOCK_READ_WAITERS); } else queue = TS_SHARED_QUEUE; - if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, - x)) { - turnstile_chain_unlock(&rw->lock_object); - v = RW_READ_VALUE(rw); - continue; - } + v |= RW_READERS_LOCK(1); + if (!atomic_fcmpset_rel_ptr(&rw->rw_lock, &v, x)) + goto retry_ts; if (LOCK_LOG_TEST(&rw->lock_object, 0)) CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", __func__, rw); @@ -983,6 +980,7 @@ __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) #endif ts = turnstile_trywait(&rw->lock_object); v = RW_READ_VALUE(rw); +retry_ts: owner = lv_rw_wowner(v); #ifdef ADAPTIVE_RWLOCKS @@ -1010,16 +1008,14 @@ __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); if ((v & ~x) == RW_UNLOCKED) { x &= ~RW_LOCK_WRITE_SPINNER; - if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) { + if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid | x)) { if (x) turnstile_claim(ts); else turnstile_cancel(ts); break; } - turnstile_cancel(ts); - v = RW_READ_VALUE(rw); - continue; + goto retry_ts; } /* * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to @@ -1027,12 +1023,9 @@ __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) * again. */ if (!(v & RW_LOCK_WRITE_WAITERS)) { - if (!atomic_cmpset_ptr(&rw->rw_lock, v, - v | RW_LOCK_WRITE_WAITERS)) { - turnstile_cancel(ts); - v = RW_READ_VALUE(rw); - continue; - } + if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, + v | RW_LOCK_WRITE_WAITERS)) + goto retry_ts; if (LOCK_LOG_TEST(&rw->lock_object, 0)) CTR2(KTR_LOCK, "%s: %p set write waiters flag", __func__, rw); diff --git a/sys/kern/kern_sx.c b/sys/kern/kern_sx.c index d5acd0ad89f9..44d0bd796d83 100644 --- a/sys/kern/kern_sx.c +++ b/sys/kern/kern_sx.c @@ -665,6 +665,7 @@ _sx_xlock_hard(struct sx *sx, uintptr_t x, int opts LOCK_FILE_LINE_ARG_DEF) sleepq_lock(&sx->lock_object); x = SX_READ_VALUE(sx); +retry_sleepq: /* * If the lock was released while spinning on the @@ -704,17 +705,13 @@ _sx_xlock_hard(struct sx *sx, uintptr_t x, int opts LOCK_FILE_LINE_ARG_DEF) * fail, restart the loop. */ if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) { - if (atomic_cmpset_acq_ptr(&sx->sx_lock, - SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS, - tid | SX_LOCK_EXCLUSIVE_WAITERS)) { - sleepq_release(&sx->lock_object); - CTR2(KTR_LOCK, "%s: %p claimed by new writer", - __func__, sx); - break; - } + if (!atomic_fcmpset_acq_ptr(&sx->sx_lock, &x, + tid | SX_LOCK_EXCLUSIVE_WAITERS)) + goto retry_sleepq; sleepq_release(&sx->lock_object); - x = SX_READ_VALUE(sx); - continue; + CTR2(KTR_LOCK, "%s: %p claimed by new writer", + __func__, sx); + break; } /* @@ -722,11 +719,9 @@ _sx_xlock_hard(struct sx *sx, uintptr_t x, int opts LOCK_FILE_LINE_ARG_DEF) * than loop back and retry. */ if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) { - if (!atomic_cmpset_ptr(&sx->sx_lock, x, + if (!atomic_fcmpset_ptr(&sx->sx_lock, &x, x | SX_LOCK_EXCLUSIVE_WAITERS)) { - sleepq_release(&sx->lock_object); - x = SX_READ_VALUE(sx); - continue; + goto retry_sleepq; } if (LOCK_LOG_TEST(&sx->lock_object, 0)) CTR2(KTR_LOCK, "%s: %p set excl waiters flag", @@ -986,7 +981,7 @@ _sx_slock_hard(struct sx *sx, int opts, uintptr_t x LOCK_FILE_LINE_ARG_DEF) */ sleepq_lock(&sx->lock_object); x = SX_READ_VALUE(sx); - +retry_sleepq: /* * The lock could have been released while we spun. * In this case loop back and retry. @@ -1019,12 +1014,9 @@ _sx_slock_hard(struct sx *sx, int opts, uintptr_t x LOCK_FILE_LINE_ARG_DEF) * back. */ if (!(x & SX_LOCK_SHARED_WAITERS)) { - if (!atomic_cmpset_ptr(&sx->sx_lock, x, - x | SX_LOCK_SHARED_WAITERS)) { - sleepq_release(&sx->lock_object); - x = SX_READ_VALUE(sx); - continue; - } + if (!atomic_fcmpset_ptr(&sx->sx_lock, &x, + x | SX_LOCK_SHARED_WAITERS)) + goto retry_sleepq; if (LOCK_LOG_TEST(&sx->lock_object, 0)) CTR2(KTR_LOCK, "%s: %p set shared waiters flag", __func__, sx);