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authorJason Low <jason.low2@hpe.com>2016-05-16 17:38:00 -0700
committerIngo Molnar <mingo@kernel.org>2016-06-03 09:47:13 +0200
commitc0fcb6c2d332041256dc55d8a1ec3c0a2d0befb8 (patch)
treeeaef72a2761e059aed127bf863e1b606c44a5da7 /kernel/locking
parente38513905eeaae59056eac2c9ac55a43b1fc41b2 (diff)
locking/rwsem: Optimize write lock by reducing operations in slowpath
When acquiring the rwsem write lock in the slowpath, we first try to set count to RWSEM_WAITING_BIAS. When that is successful, we then atomically add the RWSEM_WAITING_BIAS in cases where there are other tasks on the wait list. This causes write lock operations to often issue multiple atomic operations. We can instead make the list_is_singular() check first, and then set the count accordingly, so that we issue at most 1 atomic operation when acquiring the write lock and reduce unnecessary cacheline contention. Signed-off-by: Jason Low <jason.low2@hpe.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Waiman Long<Waiman.Long@hpe.com> Acked-by: Davidlohr Bueso <dave@stgolabs.net> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Lameter <cl@linux.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: Jason Low <jason.low2@hp.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Matt Turner <mattst88@gmail.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Richard Henderson <rth@twiddle.net> Cc: Terry Rudd <terry.rudd@hpe.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Tony Luck <tony.luck@intel.com> Link: http://lkml.kernel.org/r/1463445486-16078-2-git-send-email-jason.low2@hpe.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking')
-rw-r--r--kernel/locking/rwsem-xadd.c25
1 files changed, 18 insertions, 7 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index fcbf75ac3dcb..b957da7fcb19 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -261,17 +261,28 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
}
EXPORT_SYMBOL(rwsem_down_read_failed);
+/*
+ * This function must be called with the sem->wait_lock held to prevent
+ * race conditions between checking the rwsem wait list and setting the
+ * sem->count accordingly.
+ */
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
{
/*
- * Try acquiring the write lock. Check count first in order
- * to reduce unnecessary expensive cmpxchg() operations.
+ * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
*/
- if (count == RWSEM_WAITING_BIAS &&
- cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS,
- RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
- if (!list_is_singular(&sem->wait_list))
- rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
+ if (count != RWSEM_WAITING_BIAS)
+ return false;
+
+ /*
+ * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
+ * are other tasks on the wait list, we need to add on WAITING_BIAS.
+ */
+ count = list_is_singular(&sem->wait_list) ?
+ RWSEM_ACTIVE_WRITE_BIAS :
+ RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
+
+ if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) {
rwsem_set_owner(sem);
return true;
}