[RFC] BKL in open functions in drivers

[Alexey Klimov <klimov.linux@gmail.com>]

Hello,

Few days ago Alessio Igor Bogani<abogani@texware.it> sent me patch
that removes BKLs like lock/unlock_kernel() in open call and place mutex
there in media/radio/radio-mr800.c.
This patch broke the driver, so we figured out new approah. We added one
more mutex lock that was used in open call. The patch is below: 

diff -r ffa5df73ebeb linux/drivers/media/radio/radio-mr800.c
--- a/linux/drivers/media/radio/radio-mr800.c Fri Mar 13 00:43:34 2009
+0000
+++ b/linux/drivers/media/radio/radio-mr800.c	Thu Apr 02 00:40:56 2009
+0400
@@ -163,6 +163,7 @@
 
 	unsigned char *buffer;
 	struct mutex lock;	/* buffer locking */
+	struct mutex open;
 	int curfreq;
 	int stereo;
 	int users;
@@ -570,7 +571,7 @@
 	struct amradio_device *radio = video_get_drvdata(video_devdata(file));
 	int retval;
 
-	lock_kernel();
+	mutex_lock(&radio->open);
 
 	radio->users = 1;
 	radio->muted = 1;
@@ -580,7 +581,7 @@
 		amradio_dev_warn(&radio->videodev->dev,
 			"radio did not start up properly\n");
 		radio->users = 0;
-		unlock_kernel();
+		mutex_unlock(&radio->open);
 		return -EIO;
 	}
 
@@ -594,7 +595,7 @@
 		amradio_dev_warn(&radio->videodev->dev,
 			"set frequency failed\n");
 
-	unlock_kernel();
+	mutex_unlock(&radio->open);
 	return 0;
 }
 
@@ -735,6 +736,7 @@
 	radio->stereo = -1;
 
 	mutex_init(&radio->lock);
+	mutex_init(&radio->open);
 
 	video_set_drvdata(radio->videodev, radio);
 	retval = video_register_device(radio->videodev, VFL_TYPE_RADIO,
radio_nr);

I tested such approach using stress tool that tries to open /dev/radio0
few hundred times. Looks fine. 

So, questions are:

1) What for is lock/unlock_kernel() used in open?
2) Can it be replaced by mutex, for example?

Please, comments, exaplanations are more than welcome.

Thanks,
-- 
best regards, Klimov Alexey