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>

1 files changed, 9 insertions, 7 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index b2d9d1ac28ad..746603b42f86 100644
--- 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);