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From: Davidlohr Bueso <dave@stgolabs.net>
To: Prateek Sood <prsood@codeaurora.org>
Cc: mingo@kernel.org, longman@redhat.com, peterz@infradead.org,
	parri.andrea@gmail.com, linux-kernel@vger.kernel.org,
	sramana@codeaurora.org
Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load
Date: Wed, 20 Sep 2017 07:52:54 -0700
Message-ID: <20170920145254.GA3406@linux-80c1.suse> (raw)
In-Reply-To: <1504794658-15397-1-git-send-email-prsood@codeaurora.org>

On Thu, 07 Sep 2017, Prateek Sood wrote:
> 	/*
>+	* __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();

Instead, how about just removing the release from atomic_long_sub_return_release()
such that the osq load is not hoisted over the atomic compound (along with Peter's
comment):

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index 6c6a2141f271..487ce31078ff 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -101,7 +101,7 @@ static inline void __up_read(struct rw_semaphore *sem)
  */
 static inline void __up_write(struct rw_semaphore *sem)
 {
-	if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
+	if (unlikely(atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
 						    &sem->count) < 0))
 		rwsem_wake(sem);
 }

  parent reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-09-07 14:30 Prateek Sood
2017-09-19 14:05 ` Andrea Parri
2017-09-20 14:52 ` Davidlohr Bueso [this message]
2017-09-20 21:17   ` Andrea Parri
2017-09-27 21:20     ` Davidlohr Bueso
2017-09-26 18:37 ` Prateek Sood
2017-09-29  9:30 ` [tip:locking/urgent] locking/rwsem-xadd: Fix " tip-bot for Prateek Sood
  -- strict thread matches above, loose matches on Subject: below --
2017-08-23 11:28 [PATCH] rwsem: fix " Prateek Sood
2017-08-24 11:29 ` Peter Zijlstra
2017-08-24 12:33   ` Peter Zijlstra
2017-08-24 12:52     ` Peter Zijlstra
2017-09-07 14:08       ` 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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