linux-kernel - smp_store

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Message-ID: <CA+55aFwAjc017rHJ6pAJb5efr8a-Ydp+fCuU-e08-jaU8PhsXw@mail.gmail.com>
Date:	Wed, 1 Jul 2015 09:39:44 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: smp_store_mb() oddity..

Peter/Ingo,
 while resolving a conflict, I noticed that we have the generic
default definition of "smp_store_mb()" be:

     do { WRITE_ONCE(var, value); mb(); } while (0)

which looks pretty odd. Why? That "mb()" is a full memory barrier even
on UP, yet this is clearly a smp barrier.

So I think that "mb()" should be "smp_mb()". Looking at other
architecture definitions, most architectures already do that.

I think this is just left-over from our previous (badly specified)
"set_mb()", and that commit b92b8b35a2e3 ("locking/arch: Rename
set_mb() to smp_store_mb()") just didn't notice.  Our  old set_mb()
was already confused about whether it was a smp barrier or an IO
barrier (eg ARM uses smp_mb, x86 has separate smp/up versions, but
others dop the unconditional memory barrier).

I didn't change this in the merge, because it's not just the generic
version where the conflict was, there's also powerpc, s390 and ia64
that still have the non-smp version too. But some locking person
should probably clean this up... Hint hint,

                             Linus
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