From: Nick Piggin <nickpiggin@yahoo.com.au>
To: "Jörn Engel" <joern@wohnheim.fh-wedel.de>
Cc: Daniel Walker <dwalker@mvista.com>, Andrew Morton <akpm@osdl.org>,
linux-kernel@vger.kernel.org, hzhong@gmail.com
Subject: Re: [PATCH] Profile likely/unlikely macros
Date: Wed, 26 Apr 2006 20:07:47 +1000 [thread overview]
Message-ID: <444F4673.7060702@yahoo.com.au> (raw)
In-Reply-To: <20060426095602.GB29108@wohnheim.fh-wedel.de>
Jörn Engel wrote:
> Admitted, I'm a bit slow at times. But why does this matter?
> According to my fairly limited brain, you take a potentially expensive
> barrier, so you pay with a bit of runtime. What you buy is a smaller
> critical section, so you can save some runtime on other cpus. When
> optimizing for the common case, which is one cpu, this is a net loss.
>
> There must be some correctness issue hidden that I cannot see. Can
> you explain that to me?
Another CPU may find the bit clear, enter the critical section,
and load the old `likeliness_head' (value before being changed
by this CPU).
Then it stores the old value to likeliness->next, and overwrites
likeliness_head.
One CPU's update has now gotten lost.
(there are probably other examples, like missing likliness->type)
--
SUSE Labs, Novell Inc.
Send instant messages to your online friends http://au.messenger.yahoo.com
next prev parent reply other threads:[~2006-04-26 16:36 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-25 2:57 [PATCH] Profile likely/unlikely macros Daniel Walker
2006-04-25 3:06 ` Andrew Morton
2006-04-25 3:17 ` Daniel Walker
2006-04-25 10:11 ` Nick Piggin
2006-04-25 18:06 ` Hua Zhong
2006-04-25 18:23 ` Daniel Walker
2006-04-26 1:08 ` Nick Piggin
2006-04-26 9:56 ` Jörn Engel
2006-04-26 10:07 ` Nick Piggin [this message]
2006-04-25 23:14 ` Chen, Kenneth W
2006-04-25 9:19 ` Andreas Mohr
2006-05-01 22:05 ` Roland Dreier
-- strict thread matches above, loose matches on Subject: below --
2006-04-25 9:15 Mikael Pettersson
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