Messages in this thread Patch in this message | | | Subject | [PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock and rwsem_do_wake | From | Tim Chen <> | Date | Fri, 21 Jun 2013 16:51:35 -0700 |
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Doing cmpxchg will cause cache bouncing when checking sem->count. This could cause scalability issue in a large machine (e.g. a 80 cores box).
A pre-read of sem->count can mitigate this.
Signed-off-by: Alex Shi <alex.shi@intel.com> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com> --- include/asm-generic/rwsem.h | 8 ++++---- lib/rwsem.c | 21 +++++++++++++-------- 2 files changed, 17 insertions(+), 12 deletions(-)
diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h index bb1e2cd..052d973 100644 --- a/include/asm-generic/rwsem.h +++ b/include/asm-generic/rwsem.h @@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem) static inline int __down_write_trylock(struct rw_semaphore *sem) { - long tmp; + if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE)) + return 0; - tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE, - RWSEM_ACTIVE_WRITE_BIAS); - return tmp == RWSEM_UNLOCKED_VALUE; + return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE, + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE; } /* diff --git a/lib/rwsem.c b/lib/rwsem.c index 19c5fa9..2072af5 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -75,7 +75,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) * will block as they will notice the queued writer. */ wake_up_process(waiter->task); - goto out; + return sem; } /* Writers might steal the lock before we grant it to the next reader. @@ -85,15 +85,21 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) adjustment = 0; if (wake_type != RWSEM_WAKE_READ_OWNED) { adjustment = RWSEM_ACTIVE_READ_BIAS; - try_reader_grant: - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; - if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { - /* A writer stole the lock. Undo our reader grant. */ + while (1) { + /* A writer stole the lock. */ + if (sem->count < RWSEM_WAITING_BIAS) + return sem; + + oldcount = rwsem_atomic_update(adjustment, sem) + - adjustment; + if (likely(oldcount >= RWSEM_WAITING_BIAS)) + break; + + /* A writer stole the lock. Undo our reader grant. */ if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) - goto out; + return sem; /* Last active locker left. Retry waking readers. */ - goto try_reader_grant; } } @@ -136,7 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) sem->wait_list.next = next; next->prev = &sem->wait_list; - out: return sem; } -- 1.7.4.4
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