linux-kernel - Re: [tip:locking/core] locking/pvqspinlock: Replace xchg() by the more descriptive set

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Message-ID: <CA+55aFxre4JPFoPUC1UqsCvG8=nSka=AJMvFgqKzG8XiNWoD=A@mail.gmail.com>
Date:	Mon, 11 May 2015 10:50:42 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Douglas Hatch <doug.hatch@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Waiman Long <Waiman.Long@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Norton, Scott J" <scott.norton@...com>,
	Peter Anvin <hpa@...or.com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:locking/core] locking/pvqspinlock: Replace xchg() by the
 more descriptive set_mb()

On Mon, May 11, 2015 at 7:54 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Hmm, so I looked at the set_mb() definitions and I figure we want to do
> something like the below, right?

I don't think you need to do this for the non-smp cases. The whole
thing is about smp memory ordering, so on UP you don't even need the
WRITE_ONCE(), much less a barrier.

That said, I do wonder if we should make that "it's only an smp
barrier" more explicit. We have non-smp barriers for people who do
DMA, and while they should probably never use anything like set_mb()
anyway (they tend to want *release* semantics, not a full barrier),
from a conceptual standpoint the "set_mb()" function really is closer
to the "smp_store_release()/smp_load_acquire()" family of macros.

So I wonder if we should change the name to match.

IOW, if we are really cleaning up smp_mb() and changing most of the
lines associated with it (we really have very few users, and there
seems to be more lines *defining* smp_mb() than there are lines
*using* it in the kernel), maybe we should also just rename it
"smp_store_mb()" at the same time.

I dunno. Maybe the churn isn't worth it.

                     Linus
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