[media] Revert "[media] videobuf_vm_{open,close} race fixes"

This reverts commit a242f426108c284049a69710f871cc9f11b13e61.

That commit actually caused deadlocks, rather then fixing them.

If ext_lock is set to NULL (otherwise videobuf_queue_lock doesn't do
anything), then you get this deadlock:

The driver's mmap function calls videobuf_mmap_mapper which calls
videobuf_queue_lock on q. videobuf_mmap_mapper calls  __videobuf_mmap_mapper,
__videobuf_mmap_mapper calls videobuf_vm_open and videobuf_vm_open
calls videobuf_queue_lock on q (introduced by above patch): deadlocked.

This affects drivers using dma-contig and dma-vmalloc. Only dma-sg is
not affected since it doesn't call videobuf_vm_open from __videobuf_mmap_mapper.

Most drivers these days have a non-NULL ext_lock. Those that still use
NULL there are all fairly obscure drivers, which is why this hasn't been
seen earlier.

Since everything worked perfectly fine for many years I prefer to just
revert this patch rather than trying to fix it. videobuf is quite fragile
and I rather not touch it too much. Work is (slowly) progressing to move
everything over to vb2 or at the very least use non-NULL ext_lock in
videobuf.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: <stable@vger.kernel.org>      # for v3.11 and up
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Reported-by: Pete Eberlein <pete@sensoray.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

1 files changed, 5 insertions, 7 deletions

diff --git a/drivers/media/v4l2-core/videobuf-dma-contig.c b/drivers/media/v4l2-core/videobuf-dma-contig.c
index 65411adcd0ea..7e6b209b7002 100644
--- a/drivers/media/v4l2-core/videobuf-dma-contig.c
+++ b/drivers/media/v4l2-core/videobuf-dma-contig.c
@@ -66,14 +66,11 @@ static void __videobuf_dc_free(struct device *dev,
 static void videobuf_vm_open(struct vm_area_struct *vma)
 {
 	struct videobuf_mapping *map = vma->vm_private_data;
-	struct videobuf_queue *q = map->q;
 
-	dev_dbg(q->dev, "vm_open %p [count=%u,vma=%08lx-%08lx]\n",
+	dev_dbg(map->q->dev, "vm_open %p [count=%u,vma=%08lx-%08lx]\n",
 		map, map->count, vma->vm_start, vma->vm_end);
 
-	videobuf_queue_lock(q);
 	map->count++;
-	videobuf_queue_unlock(q);
 }
 
 static void videobuf_vm_close(struct vm_area_struct *vma)
@@ -85,11 +82,12 @@ static void videobuf_vm_close(struct vm_area_struct *vma)
 	dev_dbg(q->dev, "vm_close %p [count=%u,vma=%08lx-%08lx]\n",
 		map, map->count, vma->vm_start, vma->vm_end);
 
-	videobuf_queue_lock(q);
-	if (!--map->count) {
+	map->count--;
+	if (0 == map->count) {
 		struct videobuf_dma_contig_memory *mem;
 
 		dev_dbg(q->dev, "munmap %p q=%p\n", map, q);
+		videobuf_queue_lock(q);
 
 		/* We need first to cancel streams, before unmapping */
 		if (q->streaming)
@@ -128,8 +126,8 @@ static void videobuf_vm_close(struct vm_area_struct *vma)
 
 		kfree(map);
 
+		videobuf_queue_unlock(q);
 	}
-	videobuf_queue_unlock(q);
 }
 
 static const struct vm_operations_struct videobuf_vm_ops = {