From: Oleg Nesterov <oleg@tv-sign.ru>
To: balbir@in.ibm.com
Cc: Linus Torvalds <torvalds@osdl.org>,
Benjamin LaHaise <bcrl@kvack.org>, Ingo Molnar <mingo@elte.hu>,
Andi Kleen <ak@suse.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@osdl.org>,
Arjan van de Ven <arjanv@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
Alan Cox <alan@lxorguk.ukuu.org.uk>,
Christoph Hellwig <hch@infradead.org>,
David Howells <dhowells@redhat.com>,
Alexander Viro <viro@ftp.linux.org.uk>
Subject: Re: [patch 00/15] Generic Mutex Subsystem
Date: Wed, 11 Jan 2006 12:22:01 +0300 [thread overview]
Message-ID: <43C4CE39.3326BFBB@tv-sign.ru> (raw)
In-Reply-To: <20060111063322.GA9261@in.ibm.com>
Balbir Singh wrote:
>
> On Tue, Jan 10, 2006 at 09:03:07PM +0300, Oleg Nesterov wrote:
>
> > I don't have an answer, only a wild guess.
> >
> > Note that if P1 releases this semaphore before pre-woken P2 actually
> > gets cpu what happens is:
> >
> > P1->up() just increments ->count, no wake_up() (fastpath)
> >
> > P2 takes the semaphore without schedule.
> >
> > So *may be* it was designed this way as some form of optimization,
> > in this scenario P2 has some chances to run with sem held earlier.
> >
>
> P1->up() will do a wake_up() only if count < 0. For no wake_up()
> the count >=0 before the increment. This means that there is no one
> sleeping on the semaphore.
And this exactly happens. P1 returns from __down() with ->count == 0
and P2 pre-woken.
Oleg.
next prev parent reply other threads:[~2006-01-11 8:05 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-12-19 1:34 Ingo Molnar
2005-12-19 4:22 ` Andi Kleen
2005-12-19 4:28 ` Steven Rostedt
2005-12-19 4:31 ` Andi Kleen
2005-12-19 6:24 ` Linus Torvalds
2005-12-19 12:56 ` Steven Rostedt
2005-12-19 16:55 ` Ingo Molnar
2005-12-19 15:50 ` Ingo Molnar
2005-12-19 19:11 ` Linus Torvalds
2005-12-19 19:25 ` Benjamin LaHaise
2005-12-19 19:55 ` Linus Torvalds
2005-12-21 16:42 ` Oleg Nesterov
2006-01-10 10:28 ` Balbir Singh
2006-01-10 18:03 ` Oleg Nesterov
2006-01-11 6:33 ` Balbir Singh
2006-01-11 9:22 ` Oleg Nesterov [this message]
2005-12-19 20:11 ` Ingo Molnar
2005-12-19 20:19 ` Benjamin LaHaise
2005-12-19 20:32 ` Russell King
2005-12-19 20:57 ` Ingo Molnar
2005-12-19 19:55 ` Ingo Molnar
2005-12-19 20:12 ` Linus Torvalds
2005-12-19 23:37 ` Christoph Hellwig
2005-12-20 8:03 ` Nick Piggin
2005-12-20 8:06 ` Arjan van de Ven
2005-12-20 8:21 ` Nick Piggin
2005-12-20 8:36 ` Arjan van de Ven
2005-12-20 8:48 ` Nick Piggin
2005-12-19 16:22 ` Ingo Molnar
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