linux-kernel - Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

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Date:	Tue, 24 Jul 2007 15:32:02 +0530 (IST)
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	David Howells <dhowells@...hat.com>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered
 unnecessarily

Hi David,

On Tue, 24 Jul 2007, David Howells wrote:

> Satyam Sharma <ssatyam@....iitk.ac.in> wrote:
> 
> > OTOH, as per Linus' review it seems we can drop the "memory" clobber
> > and specify the output operand for the extended asm as "+m". But I
> > must admit I didn't quite understand that at all.
> 
> As I understand it, the "+m" indicates to the compiler a restriction on the
> ordering of things that access that particular memory location, whereas a
> "memory" indicates a restriction on the orderings of all accesses to memory -
> precisely what you need to produce a lock.

Ok, thanks -- I didn't know gcc's behaviour w.r.t. "+m" at all, but in my
defense I'd add all this was quite poorly/wrongly documented in the docs.

> There are a number of things that use test_and_set_bit() and co to implement a
> lock or other synchronisation.  This means that they must exhibit LOCK-class
> barrier effects or better.  LOCK-class barrier effects mean, more or less,
> that all memory accesses issued before the lock must happen before all memory
> accesses issued after the lock.  But it most happen at both CPU-level and
> compiler-level.  The "memory" constraint instructs the compiler in this
> regard.

Yes, thanks for laying this out so clearly, again. So combined with what
you explained above, I think I now fully understand why most of this
series was incorrect ...

> Remember also that this is gcc black magic and so some of it has had to be
> worked out empirically - possibly after sacrificing a goat under a full moon.

:-)


Satyam
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