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@xxxxxxxxx>
---
drivers/media/video/v4l2-dev.c | 37 ++++++++++++++++++++++++++++++++++---
drivers/media/video/v4l2-device.c | 1 +
include/media/v4l2-dev.h | 2 +-
include/media/v4l2-device.h | 2 ++
4 files changed, 38 insertions(+), 4 deletions(-)
diff --git a/drivers/media/video/v4l2-dev.c b/drivers/media/video/v4l2-dev.c
index 8eb0756..59ef642 100644
--- a/drivers/media/video/v4l2-dev.c
+++ b/drivers/media/video/v4l2-dev.c
@@ -258,11 +258,42 @@ 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 */
- lock_kernel();
+ /* 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;
+
+ if (cmd != VIDIOC_DQBUF) {
+ int res = mutex_lock_interruptible(m);
+
+ if (res)
+ return res;
+ }
if (video_is_registered(vdev))
ret = vdev->fops->ioctl(filp, cmd, arg);
- unlock_kernel();
+ 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
--- 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-dev.h b/include/media/v4l2-dev.h
index 15802a0..59dec5a 100644
--- a/include/media/v4l2-dev.h
+++ b/include/media/v4l2-dev.h
@@ -39,7 +39,7 @@ struct v4l2_file_operations {
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
unsigned int (*poll) (struct file *, struct poll_table_struct *);
- long (*ioctl) (struct file *, unsigned int, unsigned long);
+ long (*ioctl __deprecated) (struct file *, unsigned int, unsigned long);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct file *);
diff --git a/include/media/v4l2-device.h b/include/media/v4l2-device.h
index 6648036..b16f307 100644
--- 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.
--
1.7.0.4
--
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