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authorNeilBrown <neilb@suse.de>2011-05-10 17:49:01 +1000
committerNeilBrown <neilb@suse.de>2011-05-11 14:26:17 +1000
commitb0140891a8cea36469f58d23859e599b1122bd37 (patch)
tree01f378d9964c1d24683a3c42bfd06b1da7d985b6 /drivers/md
parent693d92a1bbc9e42681c42ed190bd42b636ca876f (diff)
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
Diffstat (limited to 'drivers/md')
-rw-r--r--drivers/md/md.c11
1 files changed, 8 insertions, 3 deletions
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 7d6f7f18a92..4a4c0f80bde 100644
--- 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) {