linux-kernel - Re: rt_mutex_timed_lock() vs hrtimer

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Message-Id: <1154436721.5932.60.camel@localhost.localdomain>
Date:	Tue, 01 Aug 2006 14:52:01 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>, Ingo Molnar <mingo@...e.hu>,
	Esben Nielsen <nielsen.esben@...glemail.com>,
	linux-kernel@...r.kernel.org
Subject: Re: rt_mutex_timed_lock() vs hrtimer_wakeup() race ?

On Tue, 2006-08-01 at 08:07 -0400, Steven Rostedt wrote:
> > We hold lock->wait_lock. The owner of this lock can be blocked itself,
> > which makes it necessary to do the chain walk. The indicator is
> > owner->pi_blocked_on. This field is only protected by owner->pi_lock.
> > 
> > If we look at this field outside of owner->pi_lock, then we might miss a
> > chain walk.
> > 
> 
> Actually Thomas, not counting the debug case, his patch wont miss a
> chain walk.  That is because the boost is read _after_ the owner's prio
> is adjusted.  So the only thing the lock is doing for us is to prevent
> us from walking the chain twice for the same lock grab. (btw. I'm
> looking at 2.6.18-rc2, and not the -rt patch, just to make things
> clear).

So what do we win, when we drop the lock before we check for boosting ?
In the worst case we do a redundant chain walk.

	tglx


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