From: Peter Zijlstra <peterz@infradead.org>
To: Prateek Sood <prsood@codeaurora.org>
Cc: mingo@redhat.com, sramana@codeaurora.org,
linux-kernel@vger.kernel.org, Waiman Long <longman@redhat.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Andrea Parri <parri.andrea@gmail.com>,
Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load
Date: Thu, 24 Aug 2017 14:33:04 +0200 [thread overview]
Message-ID: <20170824123304.5pqw53qbsytpfbrp@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170824112927.tqejopswi7mcy4sq@hirez.programming.kicks-ass.net>
On Thu, Aug 24, 2017 at 01:29:27PM +0200, Peter Zijlstra wrote:
>
> WTH did you not Cc the people that commented on your patch last time?
>
> On Wed, Aug 23, 2017 at 04:58:55PM +0530, Prateek Sood wrote:
> > If a spinner is present, there is a chance that the load of
> > rwsem_has_spinner() in rwsem_wake() can be reordered with
> > respect to decrement of rwsem count in __up_write() leading
> > to wakeup being missed.
>
> > spinning writer up_write caller
> > --------------- -----------------------
> > [S] osq_unlock() [L] osq
> > spin_lock(wait_lock)
> > sem->count=0xFFFFFFFF00000001
> > +0xFFFFFFFF00000000
> > count=sem->count
> > MB
> > sem->count=0xFFFFFFFE00000001
> > -0xFFFFFFFF00000001
> > RMB
>
> This doesn't make sense, it appears to order a STORE against something
> else.
>
> > spin_trylock(wait_lock)
> > return
> > rwsem_try_write_lock(count)
> > spin_unlock(wait_lock)
> > schedule()
Is this what you wanted to write?
---
kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 02f660666ab8..813b5d3654ce 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
DEFINE_WAKE_Q(wake_q);
/*
+ * __rwsem_down_write_failed_common(sem)
+ * rwsem_optimistic_spin(sem)
+ * osq_unlock(sem->osq)
+ * ...
+ * atomic_long_add_return(&sem->count)
+ *
+ * - VS -
+ *
+ * __up_write()
+ * if (atomic_long_sub_return_release(&sem->count) < 0)
+ * rwsem_wake(sem)
+ * osq_is_locked(&sem->osq)
+ *
+ * And __up_write() must observe !osq_is_locked() when it observes the
+ * atomic_long_add_return() in order to not miss a wakeup.
+ *
+ * This boils down to:
+ *
+ * [S.rel] X = 1 [RmW] r0 = (Y += 0)
+ * MB RMB
+ * [RmW] Y += 1 [L] r1 = X
+ *
+ * exists (r0=1 /\ r1=0)
+ */
+ smp_rmb();
+
+ /*
* If a spinner is present, it is not necessary to do the wakeup.
* Try to do wakeup only if the trylock succeeds to minimize
* spinlock contention which may introduce too much delay in the
next prev parent reply other threads:[~2017-08-24 12:33 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-23 11:28 [PATCH] rwsem: fix missed wakeup due to reordering of load Prateek Sood
2017-08-24 11:29 ` Peter Zijlstra
2017-08-24 12:33 ` Peter Zijlstra [this message]
2017-08-24 12:52 ` Peter Zijlstra
2017-09-07 14:08 ` Prateek Sood
-- strict thread matches above, loose matches on Subject: below --
2017-09-07 14:30 Prateek Sood
2017-09-19 14:05 ` Andrea Parri
2017-09-20 14:52 ` Davidlohr Bueso
2017-09-20 21:17 ` Andrea Parri
2017-09-27 21:20 ` Davidlohr Bueso
2017-09-26 18:37 ` Prateek Sood
2017-07-26 20:17 Prateek Sood
2017-07-27 15:48 ` Waiman Long
2017-07-27 16:59 ` Peter Zijlstra
2017-08-10 8:32 ` Andrea Parri
2017-08-10 10:41 ` Peter Zijlstra
2017-08-10 10:44 ` Peter Zijlstra
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