[PATCH] apm-emulation: Fix mutex race condition

[Paul Parsons <lost.distance@yahoo.com>]

A process suspending the system (e.g. apm -s) locks apm_mutex at the start of apm_ioctl() and keeps it locked while calling pm_suspend(). A second process trying to ACK the suspend (e.g. apmd) may then block in apm_ioctl() trying to lock the same mutex. The first process tries to freeze processes, but the ACK process cannot be frozen because mutex_lock() is uninterruptible. Consequently pm_suspend() will ultimately fail in try_to_freeze_tasks(). This patch allows the ACK process to be frozen if it is blocked in apm_ioctl().

Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
---
--- clean-2.6.39-rc5/drivers/char/apm-emulation.c	2011-03-15 01:20:32.000000000 +0000
+++ linux-2.6.39-rc5/drivers/char/apm-emulation.c	2011-05-03 00:34:05.467660807 +0100
@@ -275,7 +275,8 @@ apm_ioctl(struct file *filp, u_int cmd, 
 	if (!as->suser || !as->writer)
 		return -EPERM;
 
-	mutex_lock(&apm_mutex);
+	while (mutex_lock_interruptible(&apm_mutex) < 0)
+		try_to_freeze();
 	switch (cmd) {
 	case APM_IOC_SUSPEND:
 		mutex_lock(&state_lock);