linux-kernel - Re: [PATCH] percpu-rwsem: use barrier in unlock path

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Date:	Fri, 19 Oct 2012 11:48:13 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH] percpu-rwsem: use barrier in unlock path

On Thu, Oct 18, 2012 at 9:00 AM, Mikulas Patocka <mpatocka@...hat.com> wrote:
>
> What is the procedure for making changes that require support of
> architectures? It is trivial to make a patch that moves this into
> arch-specific includes, the problem is that the patch break all the
> architectures - I wrote support for x86, sparc, parisc, alpha (I can test
> those) but not the others.

We'd need to add it to everybody.

It shouldn't need per-architecture testing - since "smp_mb()" is
always safe. So we could just make all architectures default to that,
and then for x86 (and potentially others that have cheaper models for
release-consistency) just do the optimized one.

We *could* also simply do something like

   #ifndef smp_release_before_store
     #define smp_release_before_store() smp_mb()
   #endif

and basically make the rule be that only architectures that have a
cheaper one need to define it at all. That may be the correct
short-term fix, since there initially would be only a single user.

                  Linus
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