linux-kernel - [PATCH 6/6] staging: comedi: don't use mutex when polling file

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Date:	Fri,  9 Oct 2015 12:26:52 +0100
From:	Ian Abbott <abbotti@....co.uk>
To:	driverdev-devel@...uxdriverproject.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ian Abbott <abbotti@....co.uk>,
	H Hartley Sweeten <hsweeten@...ionengravers.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 6/6] 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@....co.uk>
---
 drivers/staging/comedi/comedi_fops.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 07bb197..88e9334 100644
--- 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;
 }
 
-- 
2.6.1

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