[patch 07/67] Video: pvrusb2: Solve mutex deadlock

Greg KH
2006-10-11T21:29:48Z

[-- Attachment #1: video-pvrusb2-solve-mutex-deadlock.patch --]
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-stable review patch. If anyone has any objections, please let us know.

------------------
From: Mike Isely <isely@pobox.com>

There is a mutex ordering problem between the pvrusb2 driver and the
v4l core. Two different pathways take mutexes in opposing orders and
this (under rare circumstances) can cause a deadlock. The two mutexes
in question are videodev_lock in the v4l core and device_lock inside
the pvrusb2 driver. The device_lock instance in the driver protects a
private global array of context pointers which had been implemented in
advance of v4l core changes which eliminate the video_set_drvdata()
and video_get_drvdata() functions.

This patch restores the use of video_get_drvdata() and
video_set_drvdata(), eliminating the need for the array and the mutex.
(This is actually a patch to restore the previous implementation.) We
can do this for 2.6.18 since those functions are in fact still
present. A better (and larger) solution will be done for later
kernels.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

--- a/drivers/media/video/pvrusb2/pvrusb2-v4l2.c
+++ b/drivers/media/video/pvrusb2/pvrusb2-v4l2.c
@@ -22,6 +22,7 @@

#include <linux/kernel.h>
#include <linux/version.h>
+#include <linux/videodev.h>
#include "pvrusb2-context.h"
#include "pvrusb2-hdw.h"
#include "pvrusb2.h"
@@ -35,21 +36,11 @@ struct pvr2_v4l2_dev;
struct pvr2_v4l2_fh;
struct pvr2_v4l2;

-/* V4L no longer provide the ability to set / get a private context pointer
- (i.e. video_get_drvdata / video_set_drvdata), which means we have to
- concoct our own context locating mechanism. Supposedly this is intended
- to simplify driver implementation. It's not clear to me how that can
- possibly be true. Our solution here is to maintain a lookup table of
- our context instances, indexed by the minor device number of the V4L
- device. See pvr2_v4l2_open() for some implications of this approach. */
-static struct pvr2_v4l2_dev *devices[256];
-static DEFINE_MUTEX(device_lock);

struct pvr2_v4l2_dev {
struct pvr2_v4l2 *v4lp;
struct video_device *vdev;
struct pvr2_context_stream *stream;
- int ctxt_idx;
enum pvr2_config config;
};

@@ -703,12 +694,6 @@ static void pvr2_v4l2_dev_destroy(struct
{
printk(KERN_INFO "pvrusb2: unregistering device video%d [%s]\n",
dip->vdev->minor,pvr2_config_get_name(dip->config));
- if (dip->ctxt_idx >= 0) {
- mutex_lock(&device_lock);
- devices[dip->ctxt_idx] = NULL;
- dip->ctxt_idx = -1;
- mutex_unlock(&device_lock);
- }
video_unregister_device(dip->vdev);
}

@@ -800,33 +785,10 @@ static int pvr2_v4l2_open(struct inode *
struct pvr2_v4l2 *vp;
struct pvr2_hdw *hdw;

- mutex_lock(&device_lock);
- /* MCI 7-Jun-2006 Even though we're just doing what amounts to an
- atomic read of the device mapping array here, we still need the
- mutex. The problem is that there is a tiny race possible when
- we register the device. We can't update the device mapping
- array until after the device has been registered, owing to the
- fact that we can't know the minor device number until after the
- registration succeeds. And if another thread tries to open the
- device in the window of time after registration but before the
- map is updated, then it will get back an erroneous null pointer
- and the open will result in a spurious failure. The only way to
- prevent that is to (a) be inside the mutex here before we access
- the array, and (b) cover the entire registration process later
- on with this same mutex. Thus if we get inside the mutex here,
- then we can be assured that the registration process actually
- completed correctly. This is an unhappy complication from the
- use of global data in a driver that lives in a preemptible
- environment. It sure would be nice if the video device itself
- had a means for storing and retrieving a local context pointer.
- Oh wait. It did. But now it's gone. Silly me. */
{
- unsigned int midx = iminor(file->f_dentry->d_inode);
- if (midx < sizeof(devices)/sizeof(devices[0])) {
- dip = devices[midx];
- }
+ struct video_device *vdev = video_devdata(file);
+ dip = (struct pvr2_v4l2_dev *)video_get_drvdata(vdev);
}
- mutex_unlock(&device_lock);

if (!dip) return -ENODEV; /* Should be impossible but I'm paranoid */

@@ -1066,7 +1028,7 @@ static void pvr2_v4l2_dev_init(struct pv

memcpy(dip->vdev,&vdev_template,sizeof(vdev_template));
dip->vdev->release = video_device_release;
- mutex_lock(&device_lock);
+ video_set_drvdata(dip->vdev,dip);

mindevnum = -1;
unit_number = pvr2_hdw_get_unit_number(vp->channel.mc_head->hdw);
@@ -1081,12 +1043,6 @@ static void pvr2_v4l2_dev_init(struct pv
dip->vdev->minor,pvr2_config_get_name(dip->config));
}

- if ((dip->vdev->minor < sizeof(devices)/sizeof(devices[0])) &&
- (devices[dip->vdev->minor] == NULL)) {
- dip->ctxt_idx = dip->vdev->minor;
- devices[dip->ctxt_idx] = dip;
- }
- mutex_unlock(&device_lock);

pvr2_hdw_v4l_store_minor_number(vp->channel.mc_head->hdw,
dip->vdev->minor);
@@ -1100,7 +1056,6 @@ struct pvr2_v4l2 *pvr2_v4l2_create(struc
vp = kmalloc(sizeof(*vp),GFP_KERNEL);
if (!vp) return vp;
memset(vp,0,sizeof(*vp));
- vp->video_dev.ctxt_idx = -1;
pvr2_channel_init(&vp->channel,mnp);
pvr2_trace(PVR2_TRACE_STRUCT,"Creating pvr2_v4l2 id=%p",vp);

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