ieee1394: raw1394: make write() thread-safe

Application programs should use a libraw1394 handle only in a single
thread.  The raw1394 driver was apparently relying on this, because it
did nothing to protect its fi->state variable from corruption due to
concurrent accesses.

We now serialize the fi->state accesses.  This affects the write() path.
We re-use the state_mutex which was introduced to protect fi->iso_state
accesses in the ioctl() path.  These paths and accesses are independent
of each other, hence separate mutexes could be used.  But I don't see
much benefit in that.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/ieee1394/raw1394.c

@@ -2268,6 +2268,8 @@ static ssize_t raw1394_write(struct file *file, const char __user * buffer,
 		return -EFAULT;
 	}
 
+	mutex_lock(&fi->state_mutex);
+
 	switch (fi->state) {
 	case opened:
 		retval = state_opened(fi, req);
@@ -2282,6 +2284,8 @@ static ssize_t raw1394_write(struct file *file, const char __user * buffer,
 		break;
 	}
 
+	mutex_unlock(&fi->state_mutex);
+
 	if (retval < 0) {
 		free_pending_request(req);
 	} else {