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Message-ID: <1524032458.11062.323.camel@kernel.crashing.org> Date: Wed, 18 Apr 2018 16:20:58 +1000 From: Benjamin Herrenschmidt <benh@...nel.crashing.org> To: Waiman Long <longman@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com> Cc: linux-kernel@...r.kernel.org, Dave Chinner <david@...morbit.com>, Eric Sandeen <sandeen@...hat.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Subject: Re: [PATCH] locking/rwsem: Synchronize task state & waiter->task of readers On Tue, 2018-04-10 at 13:22 -0400, Waiman Long wrote: > It was observed occasionally in PowerPC systems that there was reader > who had not been woken up but that its waiter->task had been cleared. > > One probable cause of this missed wakeup may be the fact that the > waiter->task and the task state have not been properly synchronized as > the lock release-acquire pair of different locks in the wakeup code path > does not provide a full memory barrier guarantee. So smp_store_mb() > is now used to set waiter->task to NULL to provide a proper memory > barrier for synchronization. > > Signed-off-by: Waiman Long <longman@...hat.com> That looks right... nothing in either lock or unlock will prevent a store going past a load. > --- > kernel/locking/rwsem-xadd.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index e795908..b3c588c 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -209,6 +209,23 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, > smp_store_release(&waiter->task, NULL); > } > > + /* > + * To avoid missed wakeup of reader, we need to make sure > + * that task state and waiter->task are properly synchronized. > + * > + * wakeup sleep > + * ------ ----- > + * __rwsem_mark_wake: rwsem_down_read_failed*: > + * [S] waiter->task [S] set_current_state(state) > + * MB MB > + * try_to_wake_up: > + * [L] state [L] waiter->task > + * > + * For the wakeup path, the original lock release-acquire pair > + * does not provide enough guarantee of proper synchronization. > + */ > + smp_mb(); > + > adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; > if (list_empty(&sem->wait_list)) { > /* hit end of list above */
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