1 files changed, 28 insertions, 3 deletions

diff --git a/drivers/media/video/v4l2-dev.c b/drivers/media/video/v4l2-dev.c
index bfd392e2436d..6b64fd607b20 100644
--- 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;