linux-kernel - Re: [PATCH 3/4] x86,asm: Re-work smp_store

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Message-ID: <20151027223744.GB11242@worktop.amr.corp.intel.com>
Date:	Tue, 27 Oct 2015 23:37:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

On Wed, Oct 28, 2015 at 06:33:56AM +0900, Linus Torvalds wrote:
> On Wed, Oct 28, 2015 at 4:53 AM, Davidlohr Bueso <dave@...olabs.net> wrote:
> >
> > Note that this might affect callers that could/would rely on the
> > atomicity semantics, but there are no guarantees of that for
> > smp_store_mb() mentioned anywhere, plus most archs use this anyway.
> > Thus we continue to be consistent with the memory-barriers.txt file,
> > and more importantly, maintain the semantics of the smp_ nature.
> 

> So with this patch, the whole thing becomes pointless, I feel. (Ok, so
> it may have been pointless before too, but at least before this patch
> it generated special code, now it doesn't). So why carry it along at
> all?

So I suppose this boils down to if: XCHG ends up being cheaper than
MOV+FENCE.

PeterA, any idea?
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