linux-kernel - Re: Need lockdep help

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Message-ID: <20071202120411.2aac7720@laptopd505.fenrus.org>
Date:	Sun, 2 Dec 2007 12:04:11 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Need lockdep help

On Sun, 2 Dec 2007 14:45:32 -0500 (EST)
Alan Stern <stern@...land.harvard.edu> wrote:

> Ingo:
> 
> I ran into a lockdep reporting issue just now with some new code
> under development.  I think it's a false positive; the question is
> how best to deal with it.
> 
> Here's the situation.  The new code runs during a system sleep (i.e., 
> suspend or hibernation).  Certain activities have to be deferred
> during a system sleep, so I defined an rwsem:
> system_sleep_in_progress_rwsem.
> 
> Subroutines carrying out these activities acquire a read lock on the
> rwsem, so normally they proceed with no hindrance.  During a sleep
> transition, I acquire a write lock -- this is done via a PM-notifier
> callout routine.  That is, during a PM_HIBERNATION_PREPARE or
> PM_SUSPEND_PREPARE notification the routine does down_write(), and
> during a PM_POST_HIBERNATION or PM_POST_SUSPEND notification the
> routine does up_write().
> 
> The problem is that the notifier chain itself is under the control of 
> an rwsem (to prevent the chain from being modified while it is in
> use). The resulting actions look like this:
> 
> System sleep start:
> 		down_read(notifier-chain rwsem);
> 		call the notifier routine
> 			down_write(&system_sleep_in_progress_rwsem);
> 		up_read(notifier-chain rwsem);
> 
> System sleep end:
> 		down_read(notifier-chain rwsem);
> 		call the notifier routine
> 			up_write(&system_sleep_in_progress_rwsem);
> 		up_read(notifier-chain rwsem);
> 
> This creates a lockdep violation; each rwsem in turn is locked while 
> the other is being held.  However the only way this could lead to 
> deadlock would be if there was already a bug in the system Power 
> Management code (overlapping notifications).

or.. modifications to the notifier chain while all this is happening
(remember: rwsems are fair, once a writer shows up, all readers wait)


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