staging: comedi: don't use mutex when polling file

The main mutex in a comedi device can get held for quite a while when
processing comedi instructions, so for performance reasons, the "read"
and "write" file operations do not use it; they use use the
`attach_lock` rwsemaphore to protect against the comedi device becoming
detached at an inopportune moment.  Do the same for the "poll" file
operation.

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>

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 1d6bef28582122819f52a3bb5c78c724ce08f6f2..fe25287c56885c9c07142c6f60b270b174a26c17 100644 (file)
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -2264,7 +2264,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
        struct comedi_device *dev = cfp->dev;
        struct comedi_subdevice *s, *s_read;
 
-       mutex_lock(&dev->mutex);
+       down_read(&dev->attach_lock);
 
        if (!dev->attached) {
                dev_dbg(dev->class_dev, "no driver attached\n");
@@ -2294,7 +2294,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
        }
 
 done:
-       mutex_unlock(&dev->mutex);
+       up_read(&dev->attach_lock);
        return mask;
 }