linux-kernel - Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

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Date:	Fri, 17 Jun 2016 09:46:59 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <waiman.long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper
 acquire/release barrier

On Fri, 17 Jun 2016, Davidlohr Bueso wrote:

>On Fri, 17 Jun 2016, Waiman Long wrote:
>
>>On 06/16/2016 09:11 PM, Davidlohr Bueso wrote:
>>>On Wed, 15 Jun 2016, Peter Zijlstra wrote:
>>>
>>>>Yeah, see a few patches further in this series, where he guards a
>>>>variables with the osq_lock.
>>>
>>>So one problem I have with all this is that if we are hardening 
>>>osq_lock/unlock()
>>>because of some future use that is specific to rwsems, then we 
>>>will immediately
>>>be hurting mutexes for no good reason.
>>>
>>
>>I am going to change it to use smp_acquire__after_ctrl_dep() as 
>>suggested by PeterZ. Is that a good enough compromise? I have also 
>>changed the xchg in the unlock side to xchg_release which could help 
>>performance in some archs. The thing is when developers see the name 
>>osq_lock/osq_unlock, they will naturally assume the proper barrriers 
>>are provided which is not currently the case.
>
>Oh, from your discussions with Boqun, I was under the impression that ->locked
>was now going to be properly ordered in all cases now, which is why
>I worry about mutexes.
>
>>Anyway, the change won't affect x86, it is probably ARM or PPC that 
>>may have an impact.
>
>Yes, that xchg() won't affect x86, but adding an smp_store_release(node->locked, 1)
>or such will obviously.

nm this last part, you're right, x86 smp_store_release is a nop.