diff options
| -rw-r--r-- | src/support/mtx_rw.c | 81 |
1 files changed, 51 insertions, 30 deletions
diff --git a/src/support/mtx_rw.c b/src/support/mtx_rw.c index 1c326a50318..7905241be0e 100644 --- a/src/support/mtx_rw.c +++ b/src/support/mtx_rw.c @@ -121,23 +121,22 @@ __wt_try_readlock(WT_SESSION_IMPL *session, WT_RWLOCK *l) WT_STAT_CONN_INCR(session, rwlock_read); - new.u.v = old.u.v = l->u.v; + old.u.v = l->u.v; - /* - * This read lock can only be granted if there are no active writers. - * - * Also check for overflow in case there are 64K active readers. - */ - if (old.u.s.current != old.u.s.next || - new.u.s.readers_active == UINT16_MAX) + /* This read lock can only be granted if there are no active writers. */ + if (old.u.s.current != old.u.s.next) return (EBUSY); /* * The replacement lock value is a result of adding an active reader. - * - * We rely on this atomic operation to provide a barrier. + * Check for overflow: if the maximum number of readers are already + * active, no new readers can enter the lock. */ - new.u.s.readers_active++; + new.u.v = old.u.v; + if (++new.u.s.readers_active == 0) + return (EBUSY); + + /* We rely on this atomic operation to provide a barrier. */ return (__wt_atomic_casv64(&l->u.v, old.u.v, new.u.v) ? 0 : EBUSY); } @@ -179,7 +178,8 @@ __wt_readlock(WT_SESSION_IMPL *session, WT_RWLOCK *l) new.u.v = old.u.v; /* * Check for overflow: if the maximum number of readers - * are already active, wait to try again. + * are already active, no new readers can enter the + * lock. */ if (++new.u.s.readers_active == 0) goto stall; @@ -202,8 +202,8 @@ __wt_readlock(WT_SESSION_IMPL *session, WT_RWLOCK *l) */ writers_active = old.u.s.next - old.u.s.current; if (old.u.s.readers_queued > writers_active) { -stall: __wt_cond_wait( - session, l->cond_readers, WT_THOUSAND, NULL); +stall: __wt_cond_wait(session, + l->cond_readers, 10 * WT_THOUSAND, NULL); continue; } @@ -230,8 +230,8 @@ stall: __wt_cond_wait( else { session->current_rwlock = l; session->current_rwticket = ticket; - __wt_cond_wait( - session, l->cond_readers, 0, __read_blocked); + __wt_cond_wait(session, + l->cond_readers, 10 * WT_THOUSAND, __read_blocked); } } @@ -244,7 +244,9 @@ stall: __wt_cond_wait( */ WT_READ_BARRIER(); - WT_ASSERT(session, l->u.s.readers_active > 0); + /* Sanity check that we (still) have the lock. */ + WT_ASSERT(session, + ticket == l->u.s.current && l->u.s.readers_active > 0); } /* @@ -337,26 +339,37 @@ __wt_writelock(WT_SESSION_IMPL *session, WT_RWLOCK *l) WT_STAT_CONN_INCR(session, rwlock_write); for (;;) { - new.u.v = old.u.v = l->u.v; + old.u.v = l->u.v; + + /* Allocate a ticket. */ + new.u.v = old.u.v; ticket = new.u.s.next++; /* - * Avoid wrapping: if we allocate more than 256 tickets, two - * lockers will simultaneously be granted the lock. + * Check for overflow: if the next ticket is allowed to catch + * up with the current batch, two writers could be granted the + * lock simultaneously. */ if (new.u.s.current == new.u.s.next) { - __wt_cond_wait( - session, l->cond_writers, WT_THOUSAND, NULL); + __wt_cond_wait(session, + l->cond_writers, 10 * WT_THOUSAND, NULL); continue; } if (__wt_atomic_casv64(&l->u.v, old.u.v, new.u.v)) break; } - /* Wait for our group to start and any readers to drain. */ - for (pause_cnt = 0; - ticket != l->u.s.current || l->u.s.readers_active != 0; - pause_cnt++) { + /* + * Wait for our group to start and any readers to drain. + * + * We take care here to do an atomic read of the full 64-bit lock + * value. Otherwise, reads are not guaranteed to be ordered and we + * could see no readers active from a different batch and decide that + * we have the lock. + */ + for (pause_cnt = 0, old.u.v = l->u.v; + ticket != old.u.s.current || old.u.s.readers_active != 0; + pause_cnt++, old.u.v = l->u.v) { if (pause_cnt < 1000) WT_PAUSE(); else if (pause_cnt < 1200) @@ -364,8 +377,8 @@ __wt_writelock(WT_SESSION_IMPL *session, WT_RWLOCK *l) else { session->current_rwlock = l; session->current_rwticket = ticket; - __wt_cond_wait( - session, l->cond_writers, 0, __write_blocked); + __wt_cond_wait(session, + l->cond_writers, 10 * WT_THOUSAND, __write_blocked); } } @@ -377,6 +390,10 @@ __wt_writelock(WT_SESSION_IMPL *session, WT_RWLOCK *l) * meantime. */ WT_READ_BARRIER(); + + /* Sanity check that we (still) have the lock. */ + WT_ASSERT(session, + ticket == l->u.s.current && l->u.s.readers_active == 0); } /* @@ -389,7 +406,7 @@ __wt_writeunlock(WT_SESSION_IMPL *session, WT_RWLOCK *l) WT_RWLOCK new, old; do { - new.u.v = old.u.v = l->u.v; + old.u.v = l->u.v; /* * We're holding the lock exclusive, there shouldn't be any @@ -404,6 +421,7 @@ __wt_writeunlock(WT_SESSION_IMPL *session, WT_RWLOCK *l) * to active: this could race with new readlock requests, so we * have to spin. */ + new.u.v = old.u.v; if (++new.u.s.current == new.u.s.reader) { new.u.s.readers_active = new.u.s.readers_queued; new.u.s.readers_queued = 0; @@ -426,8 +444,11 @@ __wt_writeunlock(WT_SESSION_IMPL *session, WT_RWLOCK *l) bool __wt_rwlock_islocked(WT_SESSION_IMPL *session, WT_RWLOCK *l) { + WT_RWLOCK old; + WT_UNUSED(session); - return (l->u.s.current != l->u.s.next || l->u.s.readers_active != 0); + old.u.v = l->u.v; + return (old.u.s.current != old.u.s.next || old.u.s.readers_active != 0); } #endif |