linux-kernel - Re: [PATCH v2 1/6] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW

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Date:	Thu, 16 Jul 2015 10:07:58 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>, will.deacon@....com
Subject: Re: [PATCH v2 1/6] locking/pvqspinlock: Unconditional PV kick with
 _Q_SLOW_VAL

On 07/16/2015 01:42 AM, Peter Zijlstra wrote:
> On Wed, Jul 15, 2015 at 08:18:23PM -0400, Waiman Long wrote:
>> On 07/15/2015 05:10 AM, Peter Zijlstra wrote:
>>>   	/*
>>> +	 * A failed cmpxchg doesn't provide any memory-ordering guarantees,
>>> +	 * so we need a barrier to order the read of the node data in
>>> +	 * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
>>> +	 *
>>> +	 * Matches the cmpxchg() in pv_wait_head() setting _Q_SLOW_VAL.
>>> +	 */
>>> +	smp_rmb();
>> According to memory_barriers.txt, cmpxchg() is a full memory barrier. It
>> didn't say a failed cmpxchg will lose its memory guarantee. So is the
>> documentation right?
> The documentation is not entirely clear on this; but there are hints
> that this is so.
>
>> Or is that true for some architectures? I think it is
>> not true for x86.
> On x86 LOCK CMPXCHG is always a sync point, but yes there are archs for
> which a failed cmpxchg does _NOT_ provide any barrier semantics.
>
> The reason I started looking was because Will made Argh64 one of those.

That is what I suspected. In that case, I am fine with the patch as 
smp_rmb() is an nop in x86 anyway.

Acked-by:  Waiman Long <Waiman.Long@...com>

BTW, I think we also need to update the documentation to make it clear 
that a failed cmpxchg() or atomic_cmpxchg() may not be a full memory 
barrier as most people may not be aware of that.

Cheers,
Longman
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