Commit b0140891 authored May 10, 2011 by NeilBrown

md: Fix race when creating a new md device.



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
md_alloc.

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 <neilb@suse.de>
Reported-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Tested-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Cc: stable@kernel.org

parent 693d92a1

drivers/md/md.c

+8 −3

Original line number	Diff line number	Diff line
		@@ -4347,13 +4347,19 @@ static int md_alloc(dev_t dev, char *name)
		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) {
		@@ -4367,8 +4373,7 @@ static int md_alloc(dev_t dev, char *name)
		if (mddev->kobj.sd &&
		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);
		abort:
		mutex_unlock(&disks_mutex);
		if (!error && mddev->kobj.sd) {

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