Commit c0fcb6c2 authored May 16, 2016 by Jason Low Committed by Ingo Molnar Jun 03, 2016

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&lt;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>

parent e3851390

kernel/locking/rwsem-xadd.c

+18 −7

Original line number	Diff line number	Diff line
		@@ -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;
		}