linux-kernel - [PATCH 2/3] [media] cx88: fix locking of sub-driver operations

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Date:	Sat, 2 Apr 2011 04:41:55 -0500
From:	Jonathan Nieder <jrnieder@...il.com>
To:	linux-media@...r.kernel.org
Cc:	Huber Andreas <hobrom@...ax.at>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Hans Verkuil <hverkuil@...all.nl>,
	linux-kernel@...r.kernel.org, andrew.walker27@...world.com,
	Ben Hutchings <ben@...adent.org.uk>,
	Trent Piepho <xyzzy@...akeasy.org>
Subject: [PATCH 2/3] [media] cx88: fix locking of sub-driver operations

From: Ben Hutchings <ben@...adent.org.uk>
Date: Tue, 29 Mar 2011 03:25:15 +0100

The BKL conversion of this family of drivers seems to have gone wrong.
Opening cx88-blackbird will deadlock.  Various other uses of the
sub-device and driver lists appear to be subject to race conditions.

In particular, mpeg_ops::open in the cx2388x blackbird driver acquires
the device lock and then calls the drivers' request_acquire, which
tries to acquire the lock again --- deadlock.  Fix it by clarifying
the semantics of request_acquire, request_release, advise_acquire, and
advise_release: all require the caller to hold the device lock now.

[jn: split from a larger patch, with new commit message]

Reported-by: Andi Huber <hobrom@....at>
Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=31962
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
Signed-off-by: Jonathan Nieder <jrnieder@...il.com>
Cc: stable@...nel.org
---
 drivers/media/video/cx88/cx88-blackbird.c |    9 ++-------
 drivers/media/video/cx88/cx88-dvb.c       |    8 +-------
 drivers/media/video/cx88/cx88-mpeg.c      |    4 ----
 drivers/media/video/cx88/cx88.h           |    3 ++-
 4 files changed, 5 insertions(+), 19 deletions(-)

diff --git a/drivers/media/video/cx88/cx88-blackbird.c b/drivers/media/video/cx88/cx88-blackbird.c
index 85910c6..a6f7d53 100644
--- a/drivers/media/video/cx88/cx88-blackbird.c
+++ b/drivers/media/video/cx88/cx88-blackbird.c
@@ -1125,18 +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);
-
-	/*
-	 * NEEDSWORK: the driver can be yanked from under our feet.
-	 * The following really ought to be protected with core->lock.
-	 */
-
 	if (drv)
 		drv->request_release(drv);
 
 	atomic_dec(&dev->core->mpeg_users);
 
+	mutex_unlock(&dev->core->lock);
+
 	return 0;
 }
 
diff --git a/drivers/media/video/cx88/cx88-dvb.c b/drivers/media/video/cx88/cx88-dvb.c
index 5d0f947..c69df7e 100644
--- a/drivers/media/video/cx88/cx88-dvb.c
+++ b/drivers/media/video/cx88/cx88-dvb.c
@@ -135,13 +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);
-
-	/*
-	 * NEEDSWORK: The driver can be yanked from under our feet now.
-	 * We ought to keep holding core->lock during the below.
-	 */
-
 	if (drv) {
 		if (acquire){
 			dev->frontends.active_fe_id = fe_id;
@@ -151,6 +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;
 }
diff --git a/drivers/media/video/cx88/cx88-mpeg.c b/drivers/media/video/cx88/cx88-mpeg.c
index 918172b..9147c16 100644
--- a/drivers/media/video/cx88/cx88-mpeg.c
+++ b/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;
 }
diff --git a/drivers/media/video/cx88/cx88.h b/drivers/media/video/cx88/cx88.h
index e3d56c2..9731daa 100644
--- a/drivers/media/video/cx88/cx88.h
+++ b/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 */
-- 
1.7.5.rc0

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