linux-kernel - Re: Subject: [PATCH] APM: fix deadlock in APM_IOC

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Date:	Tue, 3 Apr 2012 03:44:28 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	NeilBrown <neilb@...e.de>
Cc:	lkml <linux-kernel@...r.kernel.org>
Subject: Re: Subject: [PATCH] APM: fix deadlock in APM_IOC_SUSPEND ioctl

On Sun, 1 Apr 2012, NeilBrown wrote:

> 
> 
> I found the Xorg server on my ARM device stuck in the 'msleep()' loop
> in apm_ioctl.
> 
> I suspect it had attempted suspend immediately after resuming and lost
> a race.
> During that msleep(10);, a new suspend cycle must have started and
> changed ->suspend_state to SUSPEND_PENDING, so it was never seen to
> be SUSPEND_DONE and the loop could never exited.  It would have moved on
> to SUSPEND_ACKTO but never been able to reach SUSPEND_DONE.
> 
> So change the loop to only run while SUSPEND_ACKED rather than until
> SUSPEND_DONE.  This is much safer.
> 
> Signed-off-by: NeilBrown <neilb@...e.de>
> 
> diff --git a/drivers/char/apm-emulation.c b/drivers/char/apm-emulation.c
> index f4837a8..6005c5c 100644
> --- a/drivers/char/apm-emulation.c
> +++ b/drivers/char/apm-emulation.c
> @@ -302,7 +302,7 @@ apm_ioctl(struct file *filp, u_int cmd, u_long arg)
>  			 * anything critical, chill a bit on each iteration.
>  			 */
>  			while (wait_event_freezable(apm_suspend_waitqueue,
> -					as->suspend_state == SUSPEND_DONE))
> +					as->suspend_state != SUSPEND_ACKED))
>  				msleep(10);
>  			break;
>  		case SUSPEND_ACKTO:

Good catch Neil! I have missed this race when we were moving away from 
freezer_*_count() to wait_event_freezable() here.

Applied, thanks.

-- 
Jiri Kosina
SUSE Labs
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