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From: Waiman Long <waiman.long@hpe.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>, <manfred@colorfullife.com>,
	<mingo@kernel.org>, <torvalds@linux-foundation.org>,
	<ggherdovich@suse.com>, <mgorman@techsingularity.net>,
	<linux-kernel@vger.kernel.org>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: sem_lock() vs qspinlocks
Date: Fri, 20 May 2016 16:44:19 -0400	[thread overview]
Message-ID: <573F7723.8030201@hpe.com> (raw)
In-Reply-To: <20160520115819.GF3193@twins.programming.kicks-ass.net>

On 05/20/2016 07:58 AM, Peter Zijlstra wrote:
> On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
>> As such, the following restores the behavior of the ticket locks and 'fixes'
>> (or hides?) the bug in sems. Naturally incorrect approach:
>>
>> @@ -290,7 +290,8 @@ static void sem_wait_array(struct sem_array *sma)
>>
>> 	for (i = 0; i<  sma->sem_nsems; i++) {
>> 		sem = sma->sem_base + i;
>> -               spin_unlock_wait(&sem->lock);
>> +               while (atomic_read(&sem->lock))
>> +                       cpu_relax();
>> 	}
>> 	ipc_smp_acquire__after_spin_is_unlocked();
>> }
> The actual bug is clear_pending_set_locked() not having acquire
> semantics. And the above 'fixes' things because it will observe the old
> pending bit or the locked bit, so it doesn't matter if the store
> flipping them is delayed.

The clear_pending_set_locked() is not the only place where the lock is 
set. If there are more than one waiter, the queuing patch will be used 
instead. The set_locked(), which is also an unordered store, will then 
be used to set the lock.

Cheers,
Longman

  parent reply	other threads:[~2016-05-20 20:44 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-05-20  5:39 sem_lock() vs qspinlocks Davidlohr Bueso
2016-05-20  7:49 ` Peter Zijlstra
2016-05-20 15:00   ` Davidlohr Bueso
2016-05-20 15:05     ` Peter Zijlstra
2016-05-20 15:25       ` Davidlohr Bueso
2016-05-20 15:28       ` Peter Zijlstra
2016-05-20 20:47     ` Waiman Long
2016-05-20 20:52       ` Peter Zijlstra
2016-05-21  0:59         ` Davidlohr Bueso
2016-05-21  4:01           ` Waiman Long
2016-05-21  7:40             ` Peter Zijlstra
2016-05-20  7:53 ` Peter Zijlstra
2016-05-20  8:13 ` Peter Zijlstra
2016-05-20  8:18   ` Peter Zijlstra
2016-05-20  9:07     ` Giovanni Gherdovich
2016-05-20  9:34       ` Peter Zijlstra
2016-05-20  8:30 ` Peter Zijlstra
2016-05-20  9:00   ` Peter Zijlstra
2016-05-20 10:09     ` Ingo Molnar
2016-05-20 10:45       ` Mel Gorman
2016-05-20 11:58 ` Peter Zijlstra
2016-05-20 14:05   ` Boqun Feng
2016-05-20 15:21     ` Peter Zijlstra
2016-05-20 16:04       ` Peter Zijlstra
2016-05-20 17:00         ` Linus Torvalds
2016-05-20 21:06           ` Peter Zijlstra
2016-05-20 21:44             ` Linus Torvalds
2016-05-21  0:48               ` Davidlohr Bueso
2016-05-21  2:30                 ` Linus Torvalds
2016-05-21  7:37                 ` Peter Zijlstra
2016-05-21 13:49                   ` Manfred Spraul
2016-05-24 10:57                     ` Peter Zijlstra
2016-05-21 17:14                   ` Davidlohr Bueso
2016-05-23 12:25           ` Peter Zijlstra
2016-05-23 17:52             ` Linus Torvalds
2016-05-25  6:37               ` Boqun Feng
2016-05-22  8:43         ` Manfred Spraul
2016-05-22  9:38           ` Peter Zijlstra
2016-05-20 16:20   ` Davidlohr Bueso
2016-05-20 20:44   ` Waiman Long [this message]
2016-05-20 20:53     ` Peter Zijlstra

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