ieee1394: raw1394: fix possible deadlock in multithreaded clients

Regression in 2.6.28-rc1:  When I added the new state_mutex which
prevents corruption of raw1394's internal state when accessed by
multithreaded client applications, the following possible though
highly unlikely deadlock slipped in:

Thread A:                  Thread B:
 - acquire mmap_sem         - raw1394_write() or raw1394_ioctl()
 - raw1394_mmap()           - acquire state_mutex
 - acquire state_mutex      - copy_to/from_user(), possible page fault:
                              acquire mmap_sem

The simplest fix is to use mutex_trylock() instead of mutex_lock() in
raw1394_mmap().  This changes the behavior under contention in a way
which is visible to userspace clients.  However, since multithreaded
access was entirely buggy before state_mutex was added and libraw1394's
documentation advised application programmers to use a handle only in a
single thread, this change in behaviour should not be an issue in
practice at all.

Since we have to use mutex_trylock() in raw1394_mmap() regardless
whether /dev/raw1394 was opened with O_NONBLOCK or not, we now use
mutex_trylock() unconditionally everywhere for state_mutex, just to have
consistent behavior.
Reported-by: Johannes Weiner <hannes@saeurebad.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/ieee1394/raw1394.c

@@ -2268,7 +2268,8 @@ static ssize_t raw1394_write(struct file *file, const char __user * buffer,
 		return -EFAULT;
 	}
 
-	mutex_lock(&fi->state_mutex);
+	if (!mutex_trylock(&fi->state_mutex))
+		return -EAGAIN;
 
 	switch (fi->state) {
 	case opened:
@@ -2548,7 +2549,8 @@ static int raw1394_mmap(struct file *file, struct vm_area_struct *vma)
 	struct file_info *fi = file->private_data;
 	int ret;
 
-	mutex_lock(&fi->state_mutex);
+	if (!mutex_trylock(&fi->state_mutex))
+		return -EAGAIN;
 
 	if (fi->iso_state == RAW1394_ISO_INACTIVE)
 		ret = -EINVAL;
@@ -2669,7 +2671,8 @@ static long raw1394_ioctl(struct file *file, unsigned int cmd,
 		break;
 	}
 
-	mutex_lock(&fi->state_mutex);
+	if (!mutex_trylock(&fi->state_mutex))
+		return -EAGAIN;
 
 	switch (fi->iso_state) {
 	case RAW1394_ISO_INACTIVE: