staging: comedi: don't poll_wait on same subdevice twice

Comedi subdevices that support asynchronous acquisition commands have a
wait queue head used for blocking reads or writes and for the poll file
operation.  The comedi device may have several subdevices that support
"read" and/or "write" commands, but each open file object has at most
one "read" subdevice and one "write" subdevice.  It's possible (though
rare) for those to be the same subdevice if the subdevice supports
commands in either direction.  In that case, the "poll" file operation
doesn't really need to do a `poll_wait()` on the same subdevice twice.
Although harmless, it wastes a poll table entry.  Check for that, and
avoid it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2262,7 +2262,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	unsigned int mask = 0;
 	struct comedi_file *cfp = file->private_data;
 	struct comedi_device *dev = cfp->dev;
-	struct comedi_subdevice *s;
+	struct comedi_subdevice *s, *s_read;
 
 	mutex_lock(&dev->mutex);
 
@@ -2272,6 +2272,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	}
 
 	s = comedi_file_read_subdevice(file);
+	s_read = s;
 	if (s && s->async) {
 		poll_wait(file, &s->async->wait_head, wait);
 		if (!s->busy || !comedi_is_subdevice_running(s) ||
@@ -2284,7 +2285,8 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	if (s && s->async) {
 		unsigned int bps = comedi_bytes_per_sample(s);
 
-		poll_wait(file, &s->async->wait_head, wait);
+		if (s != s_read)
+			poll_wait(file, &s->async->wait_head, wait);
 		comedi_buf_write_alloc(s, s->async->prealloc_bufsz);
 		if (!s->busy || !comedi_is_subdevice_running(s) ||
 		    !(s->async->cmd.flags & CMDF_WRITE) ||