linux-kernel - Re: [PATCH v3 6/6] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg

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Message-ID: <20151013150427.GP21550@arm.com>
Date:	Tue, 13 Oct 2015 16:04:27 +0100
From:	Will Deacon <will.deacon@....com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v3 6/6] powerpc: atomic: Implement cmpxchg{,64}_* and
 atomic{,64}_cmpxchg_* variants

On Tue, Oct 13, 2015 at 10:58:30PM +0800, Boqun Feng wrote:
> On Tue, Oct 13, 2015 at 03:43:33PM +0100, Will Deacon wrote:
> > Putting a barrier in the middle of that critical section is probably a
> > terrible idea, and that's why I thought you were avoiding it (hence my
> 
> The fact is that I haven't thought of that way to implement
> cmpxchg_release before you ask that question ;-) And I'm not going to do
> that for now and probably not in the future.
> 
> > original question). Perhaps just add a comment to that effect, since I
> 
> Are you suggesting if I put a barrier in the middle I'd better to add a
> comment, right? So if I don't do that, it's OK to let this patch as it.

No, I mean put a comment in your file to explain the reason why you
override _relaxed and _acquire, but not _release (because overriding
_release would introduce this weird barrier in the middle of the critical
section, which would likely cause the conditional store to fail).

Will
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