linux-kernel - Re: [linux-pm] QUESTION: How to fix race between .suspend routine and watchdog timer

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Date:	Sat, 27 Oct 2007 15:17:55 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Maxim Levitsky <maximlevitsky@...il.com>
cc:	linux-kernel@...r.kernel.org, <netdev@...r.kernel.org>,
	<linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] QUESTION: How to fix race between .suspend routine
 and watchdog timer

On Fri, 26 Oct 2007, Maxim Levitsky wrote:

> > > Looking through the dmfe code, I noticed yet another possible race.
> > > A race between the .suspend, and a timer that serves both as a watchdog, and link state detector.
> > > Again I need to prevent it from running during the suspend/resume, but how?
> > > 
> > > I can use del_timer in .suspend, and mod_timer in .resume, but that doesn't protect against
> > > race with already running timer.
> > > I can use del_timer_sync, but it states that it is useless if timer re-enables itself, and I agree with that.
> > > In dmfe case the timer does re-enable itself.
> > 
> > That comment isn't right.  del_timer_sync works perfectly well even if
> > the timer routine re-enables itself, provided it stops doing so after a
> > small number of iterations.
> Thanks for the info. but....
> Due to the "don't access the hardware, while powered-off" rule, I must know that the timer isn't running.
> and won't be.
> So what function to use (if possible) to be sure that the timer won't run anymore?
> (Taking in the account the fact that it re-enables itself)

Use del_timer_sync().  It guarantees that when it returns, the timer 
will be stopped and the timer routine will no longer be running on any 
CPU.

Alan Stern

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