Commit debfab74 authored Sep 16, 2014 by Jason Low Committed by Ingo Molnar Oct 03, 2014

locking/rwsem: Avoid double checking before try acquiring write lock



Commit 9b0fc9c0 ("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 9b0fc9c0 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>

parent db0e716a

kernel/locking/rwsem-xadd.c

+11 −9

Original line number	Diff line number	Diff line
		@@ -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 &&
		/*
		* 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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