This is a note to let you know that I've just added the patch titled
md: fix a lock order reversal in md_alloc
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
md-fix-a-lock-order-reversal-in-md_alloc.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 290d0b5fc0846d83477ec64aade0d58df4fe0686
Author: Christoph Hellwig <hch@xxxxxx>
Date: Wed Sep 1 13:38:29 2021 +0200
md: fix a lock order reversal in md_alloc
[ Upstream commit 7df835a32a8bedf7ce88efcfa7c9b245b52ff139 ]
Commit b0140891a8cea3 ("md: Fix race when creating a new md device.")
not only moved assigning mddev->gendisk before calling add_disk, which
fixes the races described in the commit log, but also added a
mddev->open_mutex critical section over add_disk and creation of the
md kobj. Adding a kobject after add_disk is racy vs deleting the gendisk
right after adding it, but md already prevents against that by holding
a mddev->active reference.
On the other hand taking this lock added a lock order reversal with what
is not disk->open_mutex (used to be bdev->bd_mutex when the commit was
added) for partition devices, which need that lock for the internal open
for the partition scan, and a recent commit also takes it for
non-partitioned devices, leading to further lockdep splatter.
Fixes: b0140891a8ce ("md: Fix race when creating a new md device.")
Fixes: d62633873590 ("block: support delayed holder registration")
Reported-by: syzbot+fadc0aaf497e6a493b9f@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Tested-by: syzbot+fadc0aaf497e6a493b9f@xxxxxxxxxxxxxxxxxxxxxxxxx
Reviewed-by: NeilBrown <neilb@xxxxxxx>
Signed-off-by: Song Liu <songliubraving@xxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 761d43829b2b..c178b2f406de 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -5535,10 +5535,6 @@ static int md_alloc(dev_t dev, char *name)
*/
disk->flags |= GENHD_FL_EXT_DEVT;
mddev->gendisk = disk;
- /* As soon as we call add_disk(), another thread could get
- * through to md_open, so make sure it doesn't get too far
- */
- mutex_lock(&mddev->open_mutex);
add_disk(disk);
error = kobject_add(&mddev->kobj, &disk_to_dev(disk)->kobj, "%s", "md");
@@ -5553,7 +5549,6 @@ static int md_alloc(dev_t dev, char *name)
if (mddev->kobj.sd &&
sysfs_create_group(&mddev->kobj, &md_bitmap_group))
pr_debug("pointless warning\n");
- mutex_unlock(&mddev->open_mutex);
abort:
mutex_unlock(&disks_mutex);
if (!error && mddev->kobj.sd) {
