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

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Message-ID: <20120401201930.01202cd2@notabene.brown>
Date:	Sun, 1 Apr 2012 20:19:30 +1000
From:	NeilBrown <neilb@...e.de>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	lkml <linux-kernel@...r.kernel.org>
Subject: Subject: [PATCH] APM: fix deadlock in APM_IOC_SUSPEND ioctl

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:

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