]> git.karo-electronics.de Git - karo-tx-linux.git/commit
locking/rwsem: Avoid deceiving lock spinners
authorDavidlohr Bueso <dave@stgolabs.net>
Fri, 30 Jan 2015 09:14:26 +0000 (01:14 -0800)
committerIngo Molnar <mingo@kernel.org>
Wed, 18 Feb 2015 15:57:16 +0000 (16:57 +0100)
commitb3fd4f03ca0b9952221f39ae6790e698bf4b39e7
tree80c7076b97961309b8d3fb82fe5cae9889e07139
parent7a215f89a0335582292ec6f3edaa3abd570da75a
locking/rwsem: Avoid deceiving lock spinners

When readers hold the semaphore, the ->owner is nil. As such,
and unlike mutexes, '!owner' does not necessarily imply that
the lock is free. This will cause writers to potentially spin
excessively as they've been mislead to thinking they have a
chance of acquiring the lock, instead of blocking.

This patch therefore enhances the counter check when the owner
is not set by the time we've broken out of the loop. Otherwise
we can return true as a new owner has the lock and thus we want
to continue spinning. While at it, we can make rwsem_spin_on_owner()
less ambiguos and return right away under need_resched conditions.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/1422609267-15102-5-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
kernel/locking/rwsem-xadd.c