[PATCH] apm-emulation: Fix mutex race condition

Paul Parsons
2011-05-03T00:16:30Z

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);