From: Davide Libenzi <davidel@xmailserver.org>
To: Jonathan Lundell <jlundell@pobox.com>
Cc: Jan Hubicka <jh@suse.cz>,
linux-kernel@vger.kernel.org,
user-mode-linux-user@lists.sourceforge.net,
Jeff Dike <jdike@karaya.com>, Andrea Arcangeli <andrea@suse.de>,
Linus Torvalds <torvalds@transmeta.com>
Subject: Re: user-mode port 0.44-2.4.7
Date: Tue, 24 Jul 2001 08:41:34 -0700 (PDT) [thread overview]
Message-ID: <XFMail.20010724084134.davidel@xmailserver.org> (raw)
In-Reply-To: <p05100306b7829ca20739@[10.0.0.49]>
On 24-Jul-2001 Jonathan Lundell wrote:
> At 3:51 PM -0700 2001-07-23, Linus Torvalds wrote:
>>On Mon, 23 Jul 2001, Jonathan Lundell wrote:
>>>
>>> If jiffies were not volatile, this initializing assignment and the
>>> test at the end could be optimized away, leaving an unconditional
>>> "return 0". A lock is of no help.
>>
>>Right.
>>
>>We want optimization barriers, and there's an explicit "barrier()" thing
>>for Linux exactly for this reason.
>>
>>For historical reasons "jiffies" is actually marked volatile, but at least
>>it has reasonably good semantics with a single-data item. Which is not to
>>say I like it. But grep for "barrier()" to see how many times we make this
>>explicit in the algorithms.
>>
>>And really, THAT is my whole point. Notice in the previous mail how I used
>>"volatile" when it was part of the _algorithm_. You should not hide
>>algorithmic choices in your data structures. You should make them
>>explicit, so that when you read the code you _see_ what assumptions people
>>make.
>
> OK, sure, that's fine. Better than barrier() in some respects, too.
> Namely, 1) volatile is portable C; barrier() isn't (not that that's
> much of an issue for compiling Linux), and 2) volatile can be
> specific to a variable, unlike the indiscriminate barrier(), which
> forces a reload of everything that might be cached (OK, not a big
> deal for IA32, but nontrivial for many-register architectures). One
> could imagine a more specific barrier(jiffies) syntax, I suppose, but
> the volatile cast is nice, restricting the effect not only to the
> single variable but to the single reference to a single variable.
One more thing, with volatile you specify it one time ( declaration time ),
while with barrier() you've to spread inside the code tons of such macro
everywhere you touch the variable.
- Davide
next prev parent reply other threads:[~2001-07-24 15:38 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-07-23 5:08 user-mode port 0.44-2.4.7 Jeff Dike
2001-07-23 15:56 ` Andrea Arcangeli
2001-07-23 15:59 ` Andrea Arcangeli
2001-07-23 16:17 ` Andrea Arcangeli
2001-07-23 16:51 ` Andrea Arcangeli
2001-07-23 16:33 ` Linus Torvalds
2001-07-23 16:45 ` Andrea Arcangeli
2001-07-23 17:32 ` Linus Torvalds
2001-07-23 17:50 ` Andrea Arcangeli
2001-07-23 18:11 ` Linus Torvalds
2001-07-23 18:27 ` Andrea Arcangeli
2001-07-23 20:00 ` Linus Torvalds
2001-07-23 20:15 ` Jonathan Lundell
2001-07-23 22:51 ` Linus Torvalds
2001-07-24 3:45 ` Jonathan Lundell
2001-07-24 15:41 ` Davide Libenzi [this message]
2001-07-24 15:46 ` Alexander Viro
2001-07-24 16:01 ` Davide Libenzi
2001-07-24 16:08 ` Alexander Viro
2001-07-24 16:52 ` Davide Libenzi
2001-07-24 16:59 ` Linus Torvalds
2001-07-24 17:31 ` Davide Libenzi
2001-07-24 17:38 ` Linus Torvalds
2001-07-24 18:07 ` Anton Altaparmakov
2001-07-23 20:44 ` Chris Friesen
2001-07-23 21:11 ` Andrea Arcangeli
2001-07-23 21:50 ` Richard Gooch
2001-07-23 22:09 ` Andrea Arcangeli
2001-07-23 13:20 ` Rob Landley
2001-07-23 22:27 ` Andrea Arcangeli
2001-07-23 17:50 ` Rob Landley
2001-07-23 23:47 ` Richard Gooch
2001-07-24 0:04 ` Andrea Arcangeli
2001-07-24 9:02 ` Jan Hubicka
2001-07-24 15:35 ` Linus Torvalds
2001-07-24 16:04 ` Linus Torvalds
2001-07-25 22:49 ` Andrea Arcangeli
2001-07-25 23:16 ` Linus Torvalds
2001-07-25 23:37 ` Chris Friesen
2001-07-26 18:28 ` Jan Hubicka
2001-07-26 18:35 ` Alan Cox
2001-07-23 22:53 ` Linus Torvalds
2001-07-23 23:13 ` Alan Cox
2001-07-23 20:25 ` Jeff Dike
[not found] <no.id>
2001-07-23 20:57 ` Alan Cox
2001-07-23 21:14 ` Chris Friesen
2001-07-25 19:12 ` Alan Cox
2001-07-25 23:49 ` Alan Cox
2001-07-25 19:03 James W. Lake
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