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

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 7d6f7f18a92052e3258ffeae719c377b2df4fb78..4a4c0f80bdeb193139dafb87df925c7f760d8890 100644 (file)
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -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) {