apm-emulation: apm_mutex breaks ACK; remove it

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: NPaul Parsons <lost.distance@yahoo.com>
Acked-by: NRafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

drivers/char/apm-emulation.c

@@ -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, u_long arg)
 	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, u_long arg)
 		mutex_unlock(&state_lock);
 		break;
 	}
-	mutex_unlock(&apm_mutex);
 
 	return err;
 }
@@ -371,7 +368,6 @@ static int apm_open(struct inode * inode, struct file * filp)
 {
 	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, struct file * filp)
 
 		filp->private_data = as;
 	}
-	mutex_unlock(&apm_mutex);
 
 	return as ? 0 : -ENOMEM;
 }