From: David Howells <dhowells@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Satyam Sharma <ssatyam@cse.iitk.ac.in>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Nick Piggin <nickpiggin@yahoo.com.au>, Andi Kleen <ak@suse.de>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints
Date: Tue, 24 Jul 2007 10:22:44 +0100 [thread overview]
Message-ID: <18925.1185268964@redhat.com> (raw)
In-Reply-To: <alpine.LFD.0.999.0707231043540.3607@woody.linux-foundation.org>
Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > So, using the "+" constraint modifier for memory, like "+m" is bogus.
> > We must simply specify "=m" which handles the case correctly.
>
> No. This is wrong.
Agreed.
> "=m" means that the new value of the memory location is *set*.
>
> Which means that gcc will potentially optimize away any previous stores to
> that memory location.
>
> And yes, it happens, and yes, we've seen it.
I had a lot of "fun" with this when mucking around with the asm-optimised R/W
semaphores.
David
next prev parent reply other threads:[~2007-07-24 9:23 UTC|newest]
Thread overview: 92+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-07-23 16:05 [PATCH 0/8] i386: bitops: Cleanup, sanitize, optimize Satyam Sharma
2007-07-23 16:05 ` [PATCH 1/8] i386: bitops: Update/correct comments Satyam Sharma
2007-07-23 16:05 ` [PATCH 2/8] i386: bitops: Rectify bogus "Ir" constraints Satyam Sharma
2007-07-23 16:10 ` Andi Kleen
2007-07-23 16:21 ` Satyam Sharma
2007-07-23 16:30 ` Andi Kleen
2007-07-23 16:36 ` Jan Hubicka
2007-07-23 18:05 ` H. Peter Anvin
2007-07-23 18:28 ` H. Peter Anvin
2007-07-23 17:57 ` Linus Torvalds
2007-07-23 18:14 ` Satyam Sharma
2007-07-23 18:32 ` Andi Kleen
2007-07-23 18:39 ` H. Peter Anvin
2007-07-23 18:52 ` Satyam Sharma
2007-07-23 16:05 ` [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints Satyam Sharma
2007-07-23 16:37 ` Andi Kleen
2007-07-23 17:15 ` Satyam Sharma
2007-07-23 17:46 ` Linus Torvalds
2007-07-24 9:22 ` David Howells [this message]
2007-07-23 16:05 ` [PATCH 4/8] i386: bitops: Kill volatile-casting of memory addresses Satyam Sharma
2007-07-23 17:52 ` Linus Torvalds
2007-07-24 4:19 ` Nick Piggin
2007-07-24 6:23 ` Satyam Sharma
2007-07-24 7:16 ` Nick Piggin
2007-07-24 9:49 ` Benjamin Herrenschmidt
2007-07-24 17:20 ` Linus Torvalds
2007-07-24 17:39 ` Jeff Garzik
2007-07-25 4:54 ` Nick Piggin
2007-07-23 16:05 ` [PATCH 5/8] i386: bitops: Contain warnings fallout from the death of volatiles Satyam Sharma
2007-07-23 16:05 ` [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily Satyam Sharma
2007-07-23 16:13 ` Andi Kleen
2007-07-23 16:26 ` Satyam Sharma
2007-07-23 16:33 ` Andi Kleen
2007-07-23 17:12 ` Satyam Sharma
2007-07-23 17:49 ` Jeremy Fitzhardinge
2007-07-23 17:55 ` Linus Torvalds
2007-07-24 9:52 ` Benjamin Herrenschmidt
2007-07-24 17:24 ` Linus Torvalds
2007-07-24 17:42 ` Trond Myklebust
2007-07-24 18:13 ` Linus Torvalds
2007-07-24 18:28 ` Trond Myklebust
2007-07-24 21:37 ` Benjamin Herrenschmidt
2007-07-24 21:55 ` Trond Myklebust
2007-07-24 22:32 ` Benjamin Herrenschmidt
2007-07-25 4:10 ` Nick Piggin
2007-07-24 21:36 ` Benjamin Herrenschmidt
2007-07-24 3:57 ` Nick Piggin
2007-07-24 6:38 ` Satyam Sharma
2007-07-24 7:24 ` Nick Piggin
2007-07-24 8:29 ` Satyam Sharma
2007-07-24 8:39 ` Nick Piggin
2007-07-24 8:38 ` Trent Piepho
2007-07-24 19:39 ` Linus Torvalds
2007-07-24 20:37 ` Andi Kleen
2007-07-24 20:08 ` Linus Torvalds
2007-07-24 21:31 ` Jeremy Fitzhardinge
2007-07-24 21:46 ` Linus Torvalds
2007-07-26 1:07 ` Trent Piepho
2007-07-26 1:18 ` Linus Torvalds
2007-07-26 1:22 ` Linus Torvalds
2007-07-24 9:44 ` David Howells
2007-07-24 10:02 ` Satyam Sharma
2007-07-23 16:06 ` [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile__ Satyam Sharma
2007-07-23 16:18 ` Andi Kleen
2007-07-23 16:22 ` [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile__ II Andi Kleen
2007-07-23 16:32 ` [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile__ Satyam Sharma
2007-07-23 16:23 ` Jeremy Fitzhardinge
2007-07-23 16:43 ` Satyam Sharma
2007-07-23 17:39 ` Jeremy Fitzhardinge
2007-07-23 18:07 ` Satyam Sharma
2007-07-23 18:28 ` Jeremy Fitzhardinge
2007-07-23 20:29 ` Trent Piepho
2007-07-23 20:40 ` Jeremy Fitzhardinge
2007-07-23 21:06 ` Trent Piepho
2007-07-23 21:30 ` Andi Kleen
2007-07-23 21:48 ` Nicholas Miell
2007-07-23 16:06 ` [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear_bit() definitions Satyam Sharma
2007-07-24 3:53 ` Nick Piggin
2007-07-24 7:34 ` Satyam Sharma
2007-07-24 7:48 ` Jeremy Fitzhardinge
2007-07-24 8:31 ` Nick Piggin
2007-07-24 8:20 ` Nick Piggin
2007-07-24 9:21 ` Satyam Sharma
2007-07-24 10:25 ` Nick Piggin
2007-07-24 11:10 ` Satyam Sharma
2007-07-24 11:32 ` Nick Piggin
2007-07-24 11:45 ` Satyam Sharma
2007-07-24 12:01 ` Nick Piggin
2007-07-24 17:12 ` Linus Torvalds
2007-07-24 19:01 ` Satyam Sharma
2007-07-30 17:57 ` [PATCH 0/8] i386: bitops: Cleanup, sanitize, optimize Denis Vlasenko
2007-07-31 1:07 ` Satyam Sharma
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