[media] v4l2-dev: add flag to have the core lock all file operations

This used to be the default if the lock pointer was set, but now that lock is by
default only used for ioctl serialization. Those drivers that already used
core locking have this flag set explicitly, except for some drivers where
it was obvious that there was no need to serialize any file operations other
than ioctl.

The drivers that didn't need this flag were:

drivers/media/radio/dsbr100.c
drivers/media/radio/radio-isa.c
drivers/media/radio/radio-keene.c
drivers/media/radio/radio-miropcm20.c
drivers/media/radio/radio-mr800.c
drivers/media/radio/radio-tea5764.c
drivers/media/radio/radio-timb.c
drivers/media/video/vivi.c
sound/i2c/other/tea575x-tuner.c

The other drivers that use core locking and where it was not immediately
obvious that this flag wasn't needed were changed so that the flag is set
together with a comment that that driver needs work to avoid having to
set that flag. This will often involve taking the core lock in the fops
themselves.

Eventually this flag should go and it should not be used in new drivers.

There are a few reasons why we want to avoid core locking of non-ioctl
fops: in the case of mmap this can lead to a deadlock in rare situations
since when mmap is called the mmap_sem is held and it is possible for
other parts of the code to take that lock as well (copy_from_user()/copy_to_user()
perform a down_read(&mm->mmap_sem) when a page fault occurs).

It is very unlikely that that happens since the core lock serializes all
fops, but the kernel warns about it if lock validation is turned on.

For poll it is also undesirable to take the core lock as that can introduce
increased latency. The same is true for read/write.

While it was possible to make flags or something to turn on/off taking the
core lock for each file operation, in practice it is much simpler to just
not take it at all except for ioctl and leave it to the driver to take the
lock. There are only a handful fops compared to the zillion ioctls we have.

I also wanted to make it obvious which drivers still take the lock for all
fops, so that's why I chose to have drivers set it explicitly.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 20 insertions, 12 deletions

diff --git a/drivers/media/video/v4l2-dev.c b/drivers/media/video/v4l2-dev.c
index b1f0923..2c4feff 100644
--- a/drivers/media/video/v4l2-dev.c
+++ b/drivers/media/video/v4l2-dev.c
@@ -274,11 +274,12 @@ static ssize_t v4l2_read(struct file *filp, char __user *buf,
 
 	if (!vdev->fops->read)
 		return -EINVAL;
-	if (vdev->lock && mutex_lock_interruptible(vdev->lock))
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags) &&
+	    mutex_lock_interruptible(vdev->lock))
 		return -ERESTARTSYS;
 	if (video_is_registered(vdev))
 		ret = vdev->fops->read(filp, buf, sz, off);
-	if (vdev->lock)
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 		mutex_unlock(vdev->lock);
 	return ret;
 }
@@ -291,11 +292,12 @@ static ssize_t v4l2_write(struct file *filp, const char __user *buf,
 
 	if (!vdev->fops->write)
 		return -EINVAL;
-	if (vdev->lock && mutex_lock_interruptible(vdev->lock))
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags) &&
+	    mutex_lock_interruptible(vdev->lock))
 		return -ERESTARTSYS;
 	if (video_is_registered(vdev))
 		ret = vdev->fops->write(filp, buf, sz, off);
-	if (vdev->lock)
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 		mutex_unlock(vdev->lock);
 	return ret;
 }
@@ -307,11 +309,11 @@ static unsigned int v4l2_poll(struct file *filp, struct poll_table_struct *poll)
 
 	if (!vdev->fops->poll)
 		return DEFAULT_POLLMASK;
-	if (vdev->lock)
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 		mutex_lock(vdev->lock);
 	if (video_is_registered(vdev))
 		ret = vdev->fops->poll(filp, poll);
-	if (vdev->lock)
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 		mutex_unlock(vdev->lock);
 	return ret;
 }
@@ -399,11 +401,12 @@ static int v4l2_mmap(struct file *filp, struct vm_area_struct *vm)
 
 	if (!vdev->fops->mmap)
 		return ret;
-	if (vdev->lock && mutex_lock_interruptible(vdev->lock))
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags) &&
+	    mutex_lock_interruptible(vdev->lock))
 		return -ERESTARTSYS;
 	if (video_is_registered(vdev))
 		ret = vdev->fops->mmap(filp, vm);
-	if (vdev->lock)
+	if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 		mutex_unlock(vdev->lock);
 	return ret;
 }
@@ -426,7 +429,8 @@ static int v4l2_open(struct inode *inode, struct file *filp)
 	video_get(vdev);
 	mutex_unlock(&videodev_lock);
 	if (vdev->fops->open) {
-		if (vdev->lock && mutex_lock_interruptible(vdev->lock)) {
+		if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags) &&
+		    mutex_lock_interruptible(vdev->lock)) {
 			ret = -ERESTARTSYS;
 			goto err;
 		}
@@ -434,7 +438,7 @@ static int v4l2_open(struct inode *inode, struct file *filp)
 			ret = vdev->fops->open(filp);
 		else
 			ret = -ENODEV;
-		if (vdev->lock)
+		if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 			mutex_unlock(vdev->lock);
 	}
 
@@ -452,10 +456,10 @@ static int v4l2_release(struct inode *inode, struct file *filp)
 	int ret = 0;
 
 	if (vdev->fops->release) {
-		if (vdev->lock)
+		if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 			mutex_lock(vdev->lock);
 		vdev->fops->release(filp);
-		if (vdev->lock)
+		if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
 			mutex_unlock(vdev->lock);
 	}
 	/* decrease the refcount unconditionally since the release()
@@ -831,6 +835,10 @@ int __video_register_device(struct video_device *vdev, int type, int nr,
 	WARN_ON(video_device[vdev->minor] != NULL);
 	vdev->index = get_index(vdev);
 	mutex_unlock(&videodev_lock);
+	/* if no lock was passed, then make sure the LOCK_ALL_FOPS bit is
+	   clear and warn if it wasn't. */
+	if (vdev->lock == NULL)
+		WARN_ON(test_and_clear_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags));
 
 	if (vdev->ioctl_ops)
 		determine_valid_ioctls(vdev);