[media] V4L: improve the BKL replacement heuristic

The BKL replacement mutex had some serious performance side-effects on
V4L drivers. It is replaced by a better heuristic that works around the
worst of the side-effects.

Read the v4l2-dev.c comments for the whole sorry story. This is a
temporary measure only until we can convert all v4l drivers to use
unlocked_ioctl.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

diff --git a/drivers/media/video/v4l2-dev.c b/drivers/media/video/v4l2-dev.c
index bfd392e..6b64fd6 100644 (file)
--- a/drivers/media/video/v4l2-dev.c
+++ b/drivers/media/video/v4l2-dev.c
@@ -245,13 +245,38 @@ static long v4l2_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
                if (vdev->lock)
                        mutex_unlock(vdev->lock);
        } else if (vdev->fops->ioctl) {
-               /* TODO: convert all drivers to unlocked_ioctl */
+               /* This code path is a replacement for the BKL. It is a major
+                * hack but it will have to do for those drivers that are not
+                * yet converted to use unlocked_ioctl.
+                *
+                * There are two options: if the driver implements struct
+                * v4l2_device, then the lock defined there is used to
+                * serialize the ioctls. Otherwise the v4l2 core lock defined
+                * below is used. This lock is really bad since it serializes
+                * completely independent devices.
+                *
+                * Both variants suffer from the same problem: if the driver
+                * sleeps, then it blocks all ioctls since the lock is still
+                * held. This is very common for VIDIOC_DQBUF since that
+                * normally waits for a frame to arrive. As a result any other
+                * ioctl calls will proceed very, very slowly since each call
+                * will have to wait for the VIDIOC_QBUF to finish. Things that
+                * should take 0.01s may now take 10-20 seconds.
+                *
+                * The workaround is to *not* take the lock for VIDIOC_DQBUF.
+                * This actually works OK for videobuf-based drivers, since
+                * videobuf will take its own internal lock.
+                */
                static DEFINE_MUTEX(v4l2_ioctl_mutex);
+               struct mutex *m = vdev->v4l2_dev ?
+                       &vdev->v4l2_dev->ioctl_lock : &v4l2_ioctl_mutex;
 
-               mutex_lock(&v4l2_ioctl_mutex);
+               if (cmd != VIDIOC_DQBUF && mutex_lock_interruptible(m))
+                       return -ERESTARTSYS;
                if (video_is_registered(vdev))
                        ret = vdev->fops->ioctl(filp, cmd, arg);
-               mutex_unlock(&v4l2_ioctl_mutex);
+               if (cmd != VIDIOC_DQBUF)
+                       mutex_unlock(m);
        } else
                ret = -ENOTTY;
 

diff --git a/drivers/media/video/v4l2-device.c b/drivers/media/video/v4l2-device.c
index 0b08f96..7fe6f92 100644 (file)
--- a/drivers/media/video/v4l2-device.c
+++ b/drivers/media/video/v4l2-device.c
@@ -35,6 +35,7 @@ int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)
 
        INIT_LIST_HEAD(&v4l2_dev->subdevs);
        spin_lock_init(&v4l2_dev->lock);
+       mutex_init(&v4l2_dev->ioctl_lock);
        v4l2_dev->dev = dev;
        if (dev == NULL) {
                /* If dev == NULL, then name must be filled in by the caller */

diff --git a/include/media/v4l2-device.h b/include/media/v4l2-device.h
index 6648036..b16f307 100644 (file)
--- a/include/media/v4l2-device.h
+++ b/include/media/v4l2-device.h
@@ -51,6 +51,8 @@ struct v4l2_device {
                        unsigned int notification, void *arg);
        /* The control handler. May be NULL. */
        struct v4l2_ctrl_handler *ctrl_handler;
+       /* BKL replacement mutex. Temporary solution only. */
+       struct mutex ioctl_lock;
 };
 
 /* Initialize v4l2_dev and make dev->driver_data point to v4l2_dev.