V4L/DVB (4663): Pvrusb2: Get rid of private global context array brain damage

A previous attempt to deal with the upcoming loss of
video_set_drvdata() and video_get_drvdata() resulted in logic which
causes a circular locking dependency - also known as a deadlock.  This
changeset attacks the problem in a different manner, using a technique
that no longer requires the problematic mutex (or that private global
array either).

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

diff --git a/drivers/media/video/pvrusb2/pvrusb2-v4l2.c b/drivers/media/video/pvrusb2/pvrusb2-v4l2.c
index 156e375..f4c54ef 100644 (file)
--- a/drivers/media/video/pvrusb2/pvrusb2-v4l2.c
+++ b/drivers/media/video/pvrusb2/pvrusb2-v4l2.c
@@ -28,6 +28,7 @@
 #include "pvrusb2-debug.h"
 #include "pvrusb2-v4l2.h"
 #include "pvrusb2-ioread.h"
+#include <linux/videodev.h>
 #include <linux/videodev2.h>
 #include <media/v4l2-common.h>
 
@@ -35,21 +36,10 @@ 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 video_device devbase; /* MUST be first! */
        struct pvr2_v4l2 *v4lp;
-       struct video_device *vdev;
        struct pvr2_context_stream *stream;
-       int ctxt_idx;
        enum pvr2_config config;
 };
 
@@ -74,7 +64,7 @@ struct pvr2_v4l2 {
        struct v4l2_prio_state prio;
 
        /* streams */
-       struct pvr2_v4l2_dev video_dev;
+       struct pvr2_v4l2_dev *vdev;
 };
 
 static int video_nr[PVR_NUM] = {[0 ... PVR_NUM-1] = -1};
@@ -718,21 +708,22 @@ static int pvr2_v4l2_do_ioctl(struct inode *inode, struct file *file,
 static void pvr2_v4l2_dev_destroy(struct pvr2_v4l2_dev *dip)
 {
        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);
+              dip->devbase.minor,pvr2_config_get_name(dip->config));
+
+       /* Paranoia */
+       dip->v4lp = 0;
+       dip->stream = 0;
+
+       /* Actual deallocation happens later when all internal references
+          are gone. */
+       video_unregister_device(&dip->devbase);
 }
 
 
 static void pvr2_v4l2_destroy_no_lock(struct pvr2_v4l2 *vp)
 {
        pvr2_hdw_v4l_store_minor_number(vp->channel.mc_head->hdw,-1);
-       pvr2_v4l2_dev_destroy(&vp->video_dev);
+       pvr2_v4l2_dev_destroy(vp->vdev);
 
        pvr2_trace(PVR2_TRACE_STRUCT,"Destroying pvr2_v4l2 id=%p",vp);
        pvr2_channel_done(&vp->channel);
@@ -740,6 +731,14 @@ static void pvr2_v4l2_destroy_no_lock(struct pvr2_v4l2 *vp)
 }
 
 
+static void pvr2_video_device_release(struct video_device *vdev)
+{
+       struct pvr2_v4l2_dev *dev;
+       dev = container_of(vdev,struct pvr2_v4l2_dev,devbase);
+       kfree(dev);
+}
+
+
 static void pvr2_v4l2_internal_check(struct pvr2_channel *chp)
 {
        struct pvr2_v4l2 *vp;
@@ -811,40 +810,12 @@ static int pvr2_v4l2_release(struct inode *inode, struct file *file)
 
 static int pvr2_v4l2_open(struct inode *inode, struct file *file)
 {
-       struct pvr2_v4l2_dev *dip = NULL; /* Our own context pointer */
+       struct pvr2_v4l2_dev *dip; /* Our own context pointer */
        struct pvr2_v4l2_fh *fhp;
        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];
-               }
-       }
-       mutex_unlock(&device_lock);
-
-       if (!dip) return -ENODEV; /* Should be impossible but I'm paranoid */
+       dip = container_of(video_devdata(file),struct pvr2_v4l2_dev,devbase);
 
        vp = dip->v4lp;
        hdw = vp->channel.hdw;
@@ -1074,38 +1045,24 @@ static void pvr2_v4l2_dev_init(struct pvr2_v4l2_dev *dip,
                return;
        }
 
-       dip->vdev = video_device_alloc();
-       if (!dip->vdev) {
-               err("Alloc of pvrusb2 v4l video device failed");
-               return;
-       }
-
-       memcpy(dip->vdev,&vdev_template,sizeof(vdev_template));
-       dip->vdev->release = video_device_release;
-       mutex_lock(&device_lock);
+       memcpy(&dip->devbase,&vdev_template,sizeof(vdev_template));
+       dip->devbase.release = pvr2_video_device_release;
 
        mindevnum = -1;
        unit_number = pvr2_hdw_get_unit_number(vp->channel.mc_head->hdw);
        if ((unit_number >= 0) && (unit_number < PVR_NUM)) {
                mindevnum = video_nr[unit_number];
        }
-       if ((video_register_device(dip->vdev, v4l_type, mindevnum) < 0) &&
-           (video_register_device(dip->vdev, v4l_type, -1) < 0)) {
+       if ((video_register_device(&dip->devbase, v4l_type, mindevnum) < 0) &&
+           (video_register_device(&dip->devbase, v4l_type, -1) < 0)) {
                err("Failed to register pvrusb2 v4l video device");
        } else {
                printk(KERN_INFO "pvrusb2: registered device video%d [%s]\n",
-                      dip->vdev->minor,pvr2_config_get_name(dip->config));
+                      dip->devbase.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);
+                                       dip->devbase.minor);
 }
 
 
@@ -1116,15 +1073,19 @@ struct pvr2_v4l2 *pvr2_v4l2_create(struct pvr2_context *mnp)
        vp = kmalloc(sizeof(*vp),GFP_KERNEL);
        if (!vp) return vp;
        memset(vp,0,sizeof(*vp));
-       vp->video_dev.ctxt_idx = -1;
+       vp->vdev = kmalloc(sizeof(*vp->vdev),GFP_KERNEL);
+       if (!vp->vdev) {
+               kfree(vp);
+               return 0;
+       }
+       memset(vp->vdev,0,sizeof(*vp->vdev));
        pvr2_channel_init(&vp->channel,mnp);
        pvr2_trace(PVR2_TRACE_STRUCT,"Creating pvr2_v4l2 id=%p",vp);
 
        vp->channel.check_func = pvr2_v4l2_internal_check;
 
        /* register streams */
-       pvr2_v4l2_dev_init(&vp->video_dev,vp,pvr2_config_mpeg);
-
+       pvr2_v4l2_dev_init(vp->vdev,vp,pvr2_config_mpeg);
 
        return vp;
 }