Messages in this thread Patch in this message | | | Date | Thu, 2 Oct 2014 22:29:20 -0700 | From | tip-bot for Jason Low <> | Subject | [tip:locking/core] locking/rwsem: Avoid double checking before try acquiring write lock |
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Commit-ID: debfab74e453f079cd8b12b0604387a8c510ef3a Gitweb: http://git.kernel.org/tip/debfab74e453f079cd8b12b0604387a8c510ef3a Author: Jason Low <jason.low2@hp.com> AuthorDate: Tue, 16 Sep 2014 17:16:57 -0700 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Fri, 3 Oct 2014 06:09:29 +0200
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> --- kernel/locking/rwsem-xadd.c | 20 +++++++++++--------- 1 file changed, 11 insertions(+), 9 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 12166ec..7628c3f 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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