[media] cx88: fix locking of sub-driver operations

The BKL conversion of this driver seems to have gone wrong.
Loading the cx88-blackbird driver deadlocks.

The cause: mpeg_ops::open in the cx2388x blackbird driver acquires the
device lock and calls the sub-driver's request_acquire, which tries to
acquire the lock again.  Fix it by clarifying the semantics of
request_acquire, request_release, advise_acquire, and advise_release:
now all will rely on the caller to acquire the device lock.

Based on work by Ben Hutchings <ben@decadent.org.uk>.

Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=31962Reported-by: Andi Huber <hobrom@gmx.at>
Tested-by: Andi Huber <hobrom@gmx.at>
Tested-by: Marlon de Boer <marlon@hyves.nl>
Cc: stable@kernel.org
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/cx88/cx88-blackbird.c

@@ -1125,13 +1125,13 @@ static int mpeg_release(struct file *file)
 
 	/* Make sure we release the hardware */
 	drv = cx8802_get_driver(dev, CX88_MPEG_BLACKBIRD);
-	mutex_unlock(&dev->core->lock);
-
 	if (drv)
 		drv->request_release(drv);
 
 	atomic_dec(&dev->core->mpeg_users);
 
+	mutex_unlock(&dev->core->lock);
+
 	return 0;
 }
 

drivers/media/video/cx88/cx88-dvb.c

@@ -135,8 +135,6 @@ static int cx88_dvb_bus_ctrl(struct dvb_frontend* fe, int acquire)
 
 	mutex_lock(&dev->core->lock);
 	drv = cx8802_get_driver(dev, CX88_MPEG_DVB);
-	mutex_unlock(&dev->core->lock);
-
 	if (drv) {
 		if (acquire){
 			dev->frontends.active_fe_id = fe_id;
@@ -146,6 +144,7 @@ static int cx88_dvb_bus_ctrl(struct dvb_frontend* fe, int acquire)
 			dev->frontends.active_fe_id = 0;
 		}
 	}
+	mutex_unlock(&dev->core->lock);
 
 	return ret;
 }

drivers/media/video/cx88/cx88-mpeg.c

@@ -624,13 +624,11 @@ static int cx8802_request_acquire(struct cx8802_driver *drv)
 
 	if (drv->advise_acquire)
 	{
-		mutex_lock(&drv->core->lock);
 		core->active_ref++;
 		if (core->active_type_id == CX88_BOARD_NONE) {
 			core->active_type_id = drv->type_id;
 			drv->advise_acquire(drv);
 		}
-		mutex_unlock(&drv->core->lock);
 
 		mpeg_dbg(1,"%s() Post acquire GPIO=%x\n", __func__, cx_read(MO_GP0_IO));
 	}
@@ -643,14 +641,12 @@ static int cx8802_request_release(struct cx8802_driver *drv)
 {
 	struct cx88_core *core = drv->core;
 
-	mutex_lock(&drv->core->lock);
 	if (drv->advise_release && --core->active_ref == 0)
 	{
 		drv->advise_release(drv);
 		core->active_type_id = CX88_BOARD_NONE;
 		mpeg_dbg(1,"%s() Post release GPIO=%x\n", __func__, cx_read(MO_GP0_IO));
 	}
-	mutex_unlock(&drv->core->lock);
 
 	return 0;
 }

drivers/media/video/cx88/cx88.h

@@ -510,7 +510,8 @@ struct cx8802_driver {
 	/* Caller must _not_ hold core->lock */
 	int (*probe)(struct cx8802_driver *drv);
 
-	/* Caller must hold core->lock */
+	/* Callers to the following functions must hold core->lock */
+
 	int (*remove)(struct cx8802_driver *drv);
 
 	/* MPEG 8802 -> mini driver - Access for hardware control */