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Subject[087/146] md: Fix race when creating a new md device.
2.6.38-stable review patch.  If anyone has any objections, please let us know.


From: NeilBrown <>

commit b0140891a8cea36469f58d23859e599b1122bd37 upstream.

There is a race when creating an md device by opening /dev/mdXX.

If two processes do this at much the same time they will follow the
call path
__blkdev_get -> get_gendisk -> kobj_lookup

The first will call
-> md_probe -> md_alloc -> add_disk -> blk_register_region

and the race happens when the second gets to kobj_lookup after
add_disk has called blk_register_region but before it returns to

In the case the second will not call md_probe (as the probe is already
done) but will get a handle on the gendisk, return to __blkdev_get
which will then call md_open (via the ->open) pointer.

As mddev->gendisk hasn't been set yet, md_open will think something is
wrong an return with ERESTARTSYS.

This can loop endlessly while the first thread makes no progress
through add_disk. Nothing is blocking it, but due to scheduler
behaviour it doesn't get a turn.
So this is essentially a live-lock.

We fix this by simply moving the assignment to mddev->gendisk before
the call the add_disk() so md_open doesn't get confused.
Also move blk_queue_flush earlier because add_disk should be as late
as possible.

To make sure that md_open doesn't complete until md_alloc has done all
that is needed, we take mddev->open_mutex during the last part of
md_alloc. md_open will wait for this.

This can cause a lock-up on boot so Cc:ing for stable.
For 2.6.36 and earlier a different patch will be needed as the
'blk_queue_flush' call isn't there.

Signed-off-by: NeilBrown <>
Reported-by: Thomas Jarosch <>
Tested-by: Thomas Jarosch <>
Signed-off-by: Greg Kroah-Hartman <>

drivers/md/md.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -4335,13 +4335,19 @@ static int md_alloc(dev_t dev, char *nam
disk->fops = &md_fops;
disk->private_data = mddev;
disk->queue = mddev->queue;
+ blk_queue_flush(mddev->queue, REQ_FLUSH | REQ_FUA);
/* Allow extended partitions. This makes the
* 'mdp' device redundant, but we can't really
* remove it now.
disk->flags |= GENHD_FL_EXT_DEVT;
- add_disk(disk);
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_init_and_add(&mddev->kobj, &md_ktype,
&disk_to_dev(disk)->kobj, "%s", "md");
if (error) {
@@ -4355,8 +4361,7 @@ static int md_alloc(dev_t dev, char *nam
if (mddev-> &&
sysfs_create_group(&mddev->kobj, &md_bitmap_group))
printk(KERN_DEBUG "pointless warning\n");
- blk_queue_flush(mddev->queue, REQ_FLUSH | REQ_FUA);
+ mutex_unlock(&mddev->open_mutex);
if (!error && mddev-> {

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