dm snapshot: fix register_snapshot deadlock

register_snapshot() performs a GFP_KERNEL allocation while holding
_origins_lock for write, but that could write out dirty pages onto a
device that attempts to acquire _origins_lock for read, resulting in
deadlock.

So move the allocation up before taking the lock.

This path is not performance-critical, so it doesn't matter that we
allocate memory and free it if we find that we won't need it.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index b2d9d1ac28adb4554537fd3568ae4fe52e748a41..746603b42f86d988bef1715dfc3a5b2e51a225f8 100644 (file)
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -229,19 +229,21 @@ static void __insert_origin(struct origin *o)
  */
 static int register_snapshot(struct dm_snapshot *snap)
 {
-       struct origin *o;
+       struct origin *o, *new_o;
        struct block_device *bdev = snap->origin->bdev;
 
+       new_o = kmalloc(sizeof(*new_o), GFP_KERNEL);
+       if (!new_o)
+               return -ENOMEM;
+
        down_write(&_origins_lock);
        o = __lookup_origin(bdev);
 
-       if (!o) {
+       if (o)
+               kfree(new_o);
+       else {
                /* New origin */
-               o = kmalloc(sizeof(*o), GFP_KERNEL);
-               if (!o) {
-                       up_write(&_origins_lock);
-                       return -ENOMEM;
-               }
+               o = new_o;
 
                /* Initialise the struct */
                INIT_LIST_HEAD(&o->snapshots);