apm_mutex is locked by a process (e.g. apm -s) at the start of apm_ioctl() and remains locked while pm_suspend() is called. Any subsequent process trying to ACK the suspend (e.g. apmd) is then blocked at the start of apm_ioctl(), causing the suspend to be delayed for 5 seconds in apm_suspend_notifier() while the ACK times out. In short, ACKs don't work.
The driver's data structures are sufficiently protected by assorted locks. And pm_suspend() has its own mutex to prevent reentrancy. Consequently there is no obvious requirement for apm_mutex, which evolved from earlier BKL calls. So let's remove it.
Signed-off-by: Paul Parsons <lost.distance@xxxxxxxxx>
---
This patch supersedes my previous "apm-emulation: Fix mutex race condition" patch; this patch fixes that earlier bug too.
--- clean-2.6.39-rc7/drivers/char/apm-emulation.c 2011-03-15 01:20:32.000000000 +0000
+++ linux-2.6.39-rc7/drivers/char/apm-emulation.c 2011-05-13 01:40:02.068822695 +0100
@@ -126,7 +126,6 @@ struct apm_user {
/*
* Local variables
*/
-static DEFINE_MUTEX(apm_mutex);
static atomic_t suspend_acks_pending = ATOMIC_INIT(0);
static atomic_t userspace_notification_inhibit = ATOMIC_INIT(0);
static int apm_disabled;
@@ -275,7 +274,6 @@ apm_ioctl(struct file *filp, u_int cmd,
if (!as->suser || !as->writer)
return -EPERM;
- mutex_lock(&apm_mutex);
switch (cmd) {
case APM_IOC_SUSPEND:
mutex_lock(&state_lock);
@@ -336,7 +334,6 @@ apm_ioctl(struct file *filp, u_int cmd,
mutex_unlock(&state_lock);
break;
}
- mutex_unlock(&apm_mutex);
return err;
}
@@ -371,7 +368,6 @@ static int apm_open(struct inode * inode
{
struct apm_user *as;
- mutex_lock(&apm_mutex);
as = kzalloc(sizeof(*as), GFP_KERNEL);
if (as) {
/*
@@ -391,7 +387,6 @@ static int apm_open(struct inode * inode
filp->private_data = as;
}
- mutex_unlock(&apm_mutex);
return as ? 0 : -ENOMEM;
}
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