linux-kernel - Re: [PATCH/RFC] m68k/bitops: Make bitmap data pointer of atomic ops volatile

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Message-ID: <20110607133500.GX29924@decadent.org.uk>
Date:	Tue, 7 Jun 2011 14:35:00 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	linux-m68k@...ts.linux-m68k.org,
	Akinobu Mita <akinobu.mita@...il.com>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH/RFC] m68k/bitops: Make bitmap data pointer of atomic
 ops volatile

On Tue, Jun 07, 2011 at 01:22:29PM +0200, Arnd Bergmann wrote:
> On Tuesday 07 June 2011, Geert Uytterhoeven wrote:
> > You mean the host_busy variable in the IDE code?
> > That would also apply to context_flag in the DRM code:
> > 
> > drivers/gpu/drm/drm_context.c:233: warning: passing argument 2 of
> > ‘__constant_test_and_set_bit’ discards qualifiers from pointer target
> > type
> > drivers/gpu/drm/drm_context.c:233: warning: passing argument 2 of
> > ‘__generic_test_and_set_bit’ discards qualifiers from pointer target
> > type
> 
> Yes, that fits the same category.
> 
> > > is wrong, though. It probably doesn't hurt to do both.
> > 
> > asm-generic/bitops/atomic.h has the volatiles everywhere. That's why
> > I'm wondering.
> 
> I guess what happened is that some variables are traditionally marked
> as volatile although they shouldn't be, and most architectures have
> adapted their bitops to make the warnings go away. If you see more
> warnings of that kind, it's probably fine to just do the same on m68k.
> The volatile modifier doesn't really hurt in this case.
 
These operations are required to be atomic and therefore they
must be suitable for use with volatile-qualified variables.

Ben.

-- 
Ben Hutchings
We get into the habit of living before acquiring the habit of thinking.
                                                              - Albert Camus
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