btrfs: fix refcount_t usage when deleting btrfs_delayed_nodes

masoncl · kdave · commit ec35e48b2869 · 2018-01-02T18:00:14.000+01:00
refcounts have a generic implementation and an asm optimized one. The generic version has extra debugging to make sure that once a refcount goes to zero, refcount_inc won't increase it. The btrfs delayed inode code wasn't expecting this, and we're tripping over the warnings when the generic refcounts are used. We ended up with this race: Process A Process B btrfs_get_delayed_node() spin_lock(root->inode_lock) radix_tree_lookup() __btrfs_release_delayed_node() refcount_dec_and_test(&delayed_node->refs) our refcount is now zero refcount_add(2) <--- warning here, refcount unchanged spin_lock(root->inode_lock) radix_tree_delete() With the generic refcounts, we actually warn again when process B above tries to release his refcount because refcount_add() turned into a no-op. We saw this in production on older kernels without the asm optimized refcounts. The fix used here is to use refcount_inc_not_zero() to detect when the object is in the middle of being freed and return NULL. This is almost always the right answer anyway, since we usually end up pitching the delayed_node if it didn't have fresh data in it. This also changes __btrfs_release_delayed_node() to remove the extra check for zero refcounts before radix tree deletion. btrfs_get_delayed_node() was the only path that was allowing refcounts to go from zero to one. Fixes: 6de5f18 ("btrfs: fix refcount_t usage when deleting btrfs_delayed_node") CC: <stable@vger.kernel.org> # 4.12+ Signed-off-by: Chris Mason <clm@fb.com> Reviewed-by: Liu Bo <bo.li.liu@oracle.com> Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
@@ -87,16 +87,38 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
 
 	spin_lock(&root->inode_lock);
 	node = radix_tree_lookup(&root->delayed_nodes_tree, ino);
+
 	if (node) {
 		if (btrfs_inode->delayed_node) {
 			refcount_inc(&node->refs);	/* can be accessed */
 			BUG_ON(btrfs_inode->delayed_node != node);
 			spin_unlock(&root->inode_lock);
 			return node;
 		}
-		btrfs_inode->delayed_node = node;
-		/* can be accessed and cached in the inode */
-		refcount_add(2, &node->refs);
+
+		/*
+		 * It's possible that we're racing into the middle of removing
+		 * this node from the radix tree.  In this case, the refcount
+		 * was zero and it should never go back to one.  Just return
+		 * NULL like it was never in the radix at all; our release
+		 * function is in the process of removing it.
+		 *
+		 * Some implementations of refcount_inc refuse to bump the
+		 * refcount once it has hit zero.  If we don't do this dance
+		 * here, refcount_inc() may decide to just WARN_ONCE() instead
+		 * of actually bumping the refcount.
+		 *
+		 * If this node is properly in the radix, we want to bump the
+		 * refcount twice, once for the inode and once for this get
+		 * operation.
+		 */
+		if (refcount_inc_not_zero(&node->refs)) {
+			refcount_inc(&node->refs);
+			btrfs_inode->delayed_node = node;
+		} else {
+			node = NULL;
+		}
+
 		spin_unlock(&root->inode_lock);
 		return node;
 	}
@@ -254,17 +276,18 @@ static void __btrfs_release_delayed_node(
 	mutex_unlock(&delayed_node->mutex);
 
 	if (refcount_dec_and_test(&delayed_node->refs)) {
-		bool free = false;
 		struct btrfs_root *root = delayed_node->root;
+
 		spin_lock(&root->inode_lock);
-		if (refcount_read(&delayed_node->refs) == 0) {
-			radix_tree_delete(&root->delayed_nodes_tree,
-					  delayed_node->inode_id);
-			free = true;
-		}
+		/*
+		 * Once our refcount goes to zero, nobody is allowed to bump it
+		 * back up.  We can delete it now.
+		 */
+		ASSERT(refcount_read(&delayed_node->refs) == 0);
+		radix_tree_delete(&root->delayed_nodes_tree,
+				  delayed_node->inode_id);
 		spin_unlock(&root->inode_lock);
-		if (free)
-			kmem_cache_free(delayed_node_cache, delayed_node);
+		kmem_cache_free(delayed_node_cache, delayed_node);
 	}
 }
 
