linux-kernel - Re: [git pull] scheduler fixes

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Message-ID: <20080511111442.GA31157@parisc-linux.org>
Date:	Sun, 11 May 2008 05:14:42 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Ingo Molnar <mingo@...e.hu>,
	Sven Wegener <sven.wegener@...aler.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [git pull] scheduler fixes

On Sun, May 11, 2008 at 05:03:06AM -0600, Matthew Wilcox wrote:
> This might be the problem that causes the missing wakeups.  If you have a
> semaphore with n=2, and four processes calling down(), tasks A and B
> acquire the semaphore and tasks C and D go to sleep.  Task A calls up()
> and wakes up C.  Then task B calls up() and doesn't wake up anyone
> because C hasn't run yet.  I think we need another wakeup when task C

Er, I mis-wrote there.

Task A calls up() and wakes up C.  Then task B calls up() and wakes up C
again because C hasn't removed itself from the list yet.  D never
receives a wakeup.  The solution is for C to pass a wakeup along to the
next in line.  (The solution remains the same).

-- 
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"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
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