linux-kernel - Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write

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Message-ID: <CA+55aFwPpEOqkhi8C=ck+6WFQJHOW3LsNg-0DxjMm9m1HW1AYA@mail.gmail.com>
Date:	Sun, 19 Jan 2014 11:56:02 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Matt Turner <mattst88@...il.com>,
	Waiman Long <waiman.long@...com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Daniel J Blueman <daniel@...ascale.com>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On Sun, Jan 19, 2014 at 12:04 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> OK, another approach would be to never add "select ARCH_USE_QUEUE_RWLOCK"
> on Alpha, at least if the queued rwlocks really do want to atomically
> manipulate bytes.  After all, the Alpha systems that I know about don't
> have enough CPUs to make queued rwlocks necessary anyway.
>
> Much simpler solution!
>
> Is this what you were getting at, or am I missing your point?

You're missing something.

Just make the "writer" field be an "int" on little-endian archiectures
(like alpha).

There is no reason for that field to be a "char" to begin with, as far
as I can tell, since the padding of the structure means that it
doesn't save any space. But even if that wasn't true, we could make an
arch-specific type for "minimum type for locking".

So my *point* was that it should be easy enough to just make sure that
any data structures used for locking have types that are appropriate
for that locking.

                      Linus
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