From: Linus Torvalds <torvalds@osdl.org>
To: "linux-os (Dick Johnson)" <linux-os@analogic.com>
Cc: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@osdl.org>,
linux-kernel@vger.kernel.org, arjan@infradead.org
Subject: Re: [patch] spinlocks: remove 'volatile'
Date: Thu, 6 Jul 2006 09:07:58 -0700 (PDT) [thread overview]
Message-ID: <Pine.LNX.4.64.0607060856080.12404@g5.osdl.org> (raw)
In-Reply-To: <Pine.LNX.4.61.0607060756050.8312@chaos.analogic.com>
On Thu, 6 Jul 2006, linux-os (Dick Johnson) wrote:
>
> Then GCC must be fixed. The keyword volatile is correct. It should
> force the compiler to read the variable every time it's used.
No.
"volatile" really _is_ misdesigned. The semantics of it are so unclear as
to be totally useless. The only thing "volatile" can ever do is generate
worse code, WITH NO UPSIDES.
Historically (and from the standpoint of the C standard), the definition
of "volatile" is that any access is "visible" in the machine, and it
really kind of makes sense for hardware accesses, except these days
hardware accesses have other rules that are _not_ covered by "volatile",
so you can't actually use them for that.
And for accesses that have some software rules (ie not IO devices etc),
the rules for "volatile" are too vague to be useful.
So if you actually have rules about how to access a particular piece of
memory, just make those rules _explicit_. Use the real rules. Not
volatile, because volatile will always do the wrong thing.
Also, more importantly, "volatile" is on the wrong _part_ of the whole
system. In C, it's "data" that is volatile, but that is insane. Data
isn't volatile - _accesses_ are volatile. So it may make sense to say
"make this particular _access_ be careful", but not "make all accesses to
this data use some random strategy".
So the only thing "volatile" is potentially useful for is:
- actual accessor functions can use it in a _cast_ to make one particular
access follow the rules of "don't cache this one dereference". That is
useful as part of a _bigger_ set of rules about that access (ie it
might be the internal implementation of a "readb()", for example).
- for "random number generation" data locations, where you literally
don't _have_ any rules except "it's a random number". The only really
valid example of this is the "jiffy" timer tick.
Any other use of "volatile" is almost certainly a bug, or just useless.
It's a bug if the volatile means that you don't follow the proper protocol
for accessing the data, and it's useless (and generally generates worse
code) if you already do.
So just say NO! to volatile except under the above circumstances.
Linus
next prev parent reply other threads:[~2006-07-06 16:08 UTC|newest]
Thread overview: 163+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-07-05 8:49 [patch] uninline init_waitqueue_*() functions Ingo Molnar
2006-07-05 9:31 ` Andrew Morton
2006-07-05 9:32 ` Ingo Molnar
2006-07-05 9:53 ` Andrew Morton
2006-07-05 10:26 ` Ingo Molnar
2006-07-05 11:30 ` Ingo Molnar
2006-07-05 11:46 ` Ingo Molnar
2006-07-05 17:10 ` Andrew Morton
2006-07-05 19:35 ` Ingo Molnar
2006-07-05 20:18 ` Andrew Morton
2006-07-05 20:36 ` Linus Torvalds
2006-07-05 20:47 ` Ingo Molnar
2006-07-05 21:15 ` Ingo Molnar
2006-07-05 21:21 ` Linus Torvalds
2006-07-05 21:45 ` Ingo Molnar
2006-07-05 21:58 ` Randy.Dunlap
2006-07-05 22:00 ` Ingo Molnar
2006-07-05 22:10 ` Randy.Dunlap
2006-07-05 22:27 ` Ingo Molnar
2006-07-05 23:15 ` Adrian Bunk
2006-07-05 22:09 ` Linus Torvalds
2006-07-05 22:30 ` Ingo Molnar
2006-07-05 23:11 ` Linus Torvalds
2006-07-06 8:16 ` [patch] spinlocks: remove 'volatile' Ingo Molnar
2006-07-06 8:23 ` Ingo Molnar
2006-07-06 9:27 ` Heiko Carstens
2006-07-06 9:34 ` Arjan van de Ven
2006-07-06 11:59 ` linux-os (Dick Johnson)
2006-07-06 12:01 ` Arjan van de Ven
2006-07-06 12:29 ` linux-os (Dick Johnson)
2006-07-06 12:39 ` Arjan van de Ven
2006-07-06 13:39 ` J.A. Magallón
2006-07-06 13:43 ` Arjan van de Ven
2006-07-06 14:05 ` Chase Venters
2006-07-06 14:26 ` Andreas Schwab
2006-07-06 16:40 ` Nick Piggin
2006-07-06 23:19 ` David Schwartz
2006-07-06 18:15 ` Mark Lord
2006-07-06 19:15 ` Linus Torvalds
2006-07-06 19:33 ` Chris Friesen
2006-07-06 19:37 ` Mark Lord
2006-07-06 20:28 ` Chris Friesen
2006-07-06 20:32 ` Linus Torvalds
2006-07-06 19:38 ` Arjan van de Ven
2006-07-06 19:41 ` Måns Rullgård
2006-07-06 19:42 ` Jeff Garzik
2006-07-06 19:58 ` Linus Torvalds
2006-07-06 20:27 ` Mark Lord
2006-07-06 20:40 ` Chris Friesen
2006-07-06 21:00 ` Linus Torvalds
2006-07-08 8:40 ` Avi Kivity
2006-07-08 8:51 ` Arjan van de Ven
2006-07-08 9:20 ` Avi Kivity
2006-07-08 9:51 ` Arjan van de Ven
2006-07-08 10:18 ` Avi Kivity
2006-07-08 10:28 ` Thomas Gleixner
2006-07-09 4:19 ` Benjamin Herrenschmidt
2006-07-09 12:47 ` Avi Kivity
2006-07-09 19:16 ` David Schwartz
2006-07-09 19:51 ` Theodore Tso
2006-07-09 20:40 ` [OT] 'volatile' in userspace Rutger Nijlunsing
2006-07-10 3:42 ` Theodore Tso
2006-07-10 17:00 ` Joshua Hudson
2006-07-10 17:54 ` Nick Piggin
2006-07-11 7:48 ` Jan Engelhardt
2006-07-11 11:53 ` Nick Piggin
2006-07-11 19:09 ` Björn Steinbrink
2006-07-11 20:55 ` Jeff Dike
2006-07-10 18:54 ` Jeff Dike
2006-07-10 20:09 ` Philippe Troin
2006-07-10 20:52 ` Jeff Dike
2006-07-06 16:07 ` Linus Torvalds [this message]
2006-07-06 16:13 ` [patch] spinlocks: remove 'volatile' Linus Torvalds
2006-07-06 17:04 ` Jeff Garzik
2006-07-06 17:52 ` linux-os (Dick Johnson)
2006-07-06 18:00 ` Linus Torvalds
2006-07-06 21:02 ` J.A. Magallón
2006-07-06 21:12 ` Linus Torvalds
2006-07-06 18:10 ` Michael Buesch
2006-07-06 18:16 ` Chase Venters
2006-07-07 18:16 ` Krzysztof Halasa
2006-07-07 19:51 ` linux-os (Dick Johnson)
2006-07-07 20:25 ` Linus Torvalds
2006-07-07 21:22 ` linux-os (Dick Johnson)
2006-07-07 21:48 ` Chase Venters
2006-07-08 10:00 ` Krzysztof Halasa
2006-07-08 13:41 ` Chase Venters
2006-07-08 20:09 ` Krzysztof Halasa
2006-07-08 20:40 ` Chase Venters
2006-07-08 20:47 ` Chase Venters
2006-07-09 10:57 ` Krzysztof Halasa
2006-07-07 21:59 ` Linus Torvalds
2006-07-07 22:06 ` Linus Torvalds
2006-07-08 20:49 ` Pavel Machek
2006-07-08 21:43 ` Linus Torvalds
2006-07-07 22:05 ` J.A. Magallón
2006-07-07 22:22 ` Chase Venters
2006-07-07 22:37 ` J.A. Magallón
2006-07-08 9:33 ` David Schwartz
2006-07-07 22:49 ` J.A. Magallón
2006-07-07 22:59 ` Vadim Lobanov
2006-07-07 23:18 ` Chase Venters
2006-07-07 23:36 ` Davide Libenzi
2006-07-07 22:09 ` Ingo Molnar
2006-07-08 7:36 ` Ingo Molnar
2006-07-07 20:39 ` Krzysztof Halasa
2006-07-07 23:06 ` Björn Steinbrink
2006-07-08 8:36 ` Avi Kivity
2006-07-06 19:32 ` Jan Engelhardt
2006-07-06 20:26 ` Jeremy Fitzhardinge
2006-07-06 20:55 ` Jan Engelhardt
2006-07-06 21:07 ` Linus Torvalds
2006-07-06 19:56 ` Linus Torvalds
2006-07-06 20:34 ` Linus Torvalds
2006-07-08 22:50 ` Ralf Baechle
2006-07-09 3:09 ` Linus Torvalds
2006-07-09 3:07 ` Keith Owens
2006-07-09 3:29 ` Linus Torvalds
2006-07-09 3:43 ` Keith Owens
2006-07-09 3:58 ` Linus Torvalds
2006-07-09 6:13 ` David Miller
2006-07-09 14:28 ` Roman Zippel
2006-07-09 15:27 ` Linus Torvalds
2006-07-06 8:18 ` [patch] lockdep: clean up completion initializer in smpboot.c Ingo Molnar
2006-07-06 8:23 ` [patch] uninline init_waitqueue_*() functions Ingo Molnar
2006-07-06 9:02 ` Andrew Morton
2006-07-06 9:12 ` [patch] uninline init_waitqueue_head() Ingo Molnar
2006-07-05 20:45 ` [patch] uninline init_waitqueue_*() functions Ingo Molnar
2006-07-05 10:37 ` Christoph Hellwig
2006-07-05 10:44 ` Andrew Morton
2006-07-05 10:46 ` Andrew Morton
2006-07-05 10:47 ` Ingo Molnar
2006-07-05 9:54 ` [patch] uninline init_waitqueue_*() functions, fix Ingo Molnar
2006-07-07 10:21 [patch] spinlocks: remove 'volatile' Chuck Ebbert
2006-07-07 17:09 ` Linus Torvalds
2006-07-08 3:54 Albert Cahalan
2006-07-08 5:25 ` Linus Torvalds
2006-07-08 6:39 ` Nick Piggin
2006-07-08 18:53 ` Linus Torvalds
2006-07-08 9:45 ` Joe Korty
2006-07-08 9:52 ` Arjan van de Ven
2006-07-08 9:59 ` David Schwartz
2006-07-08 10:24 ` Thomas Gleixner
2006-07-08 15:49 ` Albert Cahalan
2006-07-08 18:31 ` Thomas Gleixner
2006-07-08 19:33 ` Albert Cahalan
2006-07-08 20:11 ` Linus Torvalds
2006-07-09 21:10 ` Pavel Machek
2006-07-09 22:18 ` Linus Torvalds
2006-07-10 16:25 ` marty fouts
2006-07-08 23:10 ` David Schwartz
2006-07-08 13:57 ` Andi Kleen
2006-07-08 19:13 ` Linus Torvalds
2006-07-08 10:45 ` Krzysztof Halasa
2006-07-08 6:12 trajce nedev
2006-07-08 6:19 ` Chase Venters
2006-07-08 6:45 ` trajce nedev
2006-07-08 6:58 ` Arjan van de Ven
2006-07-08 7:02 ` Vadim Lobanov
2006-07-08 13:46 ` Chase Venters
2006-07-09 4:39 ` Benjamin Herrenschmidt
2006-07-14 3:30 ` Steven Rostedt
2006-07-08 18:23 ` Linus Torvalds
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