linux-kernel - Re: [PATCH][RFC]: mutex: adaptive spin

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Message-ID: <alpine.DEB.1.10.0901061401270.10871@gandalf.stny.rr.com>
Date:	Tue, 6 Jan 2009 14:03:02 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gregory Haskins <ghaskins@...ell.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH][RFC]: mutex: adaptive spin


> 
> Btw, this also fixes a bug: your patch did
> 
> 	+                     __set_task_state(task, state);
> 	+                     /* didnt get the lock, go to sleep: */
> 	+                     schedule();
> 
> for the schedule case without holding the mutex spinlock.
> 
> And that seems very buggy and racy indeed: since it doesn't hold the mutex 
> lock, if the old owner releases the mutex at just the right point (yeah, 
> yeah, it requires a scheduling event on another CPU in order to also miss 
> the whole "task_is_current()" logic), the wakeup can get lost, because you 
> set the state to sleeping perhaps _after_ the task just got woken up. So 
> we stay sleeping even though the mutex is clear.


That is indeed a bug.

Peter, why did you need to move __set_task_state down here?  The -rt patch 
does not do this.

-- Steve
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