From: Mikulas Patocka <mpatocka@redhat.com>
Date: Thu, 30 Oct 2008 13:33:12 +0000 (+0000)
Subject: dm snapshot: fix register_snapshot deadlock
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=60c856c8e2f57a3f69c505735ef66e3719ea0bd6;p=GitHub%2FLineageOS%2FG12%2Fandroid_kernel_amlogic_linux-4.9.git

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 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);