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authorJason Low <jason.low2@hp.com>2014-09-16 17:16:57 -0700
committerIngo Molnar <mingo@kernel.org>2014-10-03 06:09:29 +0200
commitdebfab74e453f079cd8b12b0604387a8c510ef3a (patch)
treebab0d5cc8f56dde5ad91d409683aac5d7fec8b47
parentdb0e716a1512179e8374a74c1f3184e9ce15d138 (diff)
locking/rwsem: Avoid double checking before try acquiring write lock
Commit 9b0fc9c09f1b ("rwsem: skip initial trylock in rwsem_down_write_failed") checks for if there are known active lockers in order to avoid write trylocking using expensive cmpxchg() when it likely wouldn't get the lock. However, a subsequent patch was added such that we directly check for sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg(). Thus, commit 9b0fc9c09f1b now just adds overhead. This patch modifies it so that we only do a check for if count == RWSEM_WAITING_BIAS. Also, add a comment on why we do an "extra check" of count before the cmpxchg(). Signed-off-by: Jason Low <jason.low2@hp.com> Acked-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Chegu Vinod <chegu_vinod@hp.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1410913017.2447.22.camel@j-VirtualBox Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--kernel/locking/rwsem-xadd.c20
1 files changed, 11 insertions, 9 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 12166ec9b7e7..7628c3fc37ca 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -250,16 +250,18 @@ EXPORT_SYMBOL(rwsem_down_read_failed);
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
{
- if (!(count & RWSEM_ACTIVE_MASK)) {
- /* try acquiring the write lock */
- if (sem->count == RWSEM_WAITING_BIAS &&
- cmpxchg(&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);
- return true;
- }
+ /*
+ * Try acquiring the write lock. Check count first in order
+ * to reduce unnecessary expensive cmpxchg() operations.
+ */
+ if (count == RWSEM_WAITING_BIAS &&
+ cmpxchg(&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);
+ return true;
}
+
return false;
}

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