FAILED: patch "[PATCH] btrfs: do not call synchronize_srcu() in inode_tree_del" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f72ff01df9cf5db25c76674cac16605992d15467 Mon Sep 17 00:00:00 2001
From: Josef Bacik <[email protected]>
Date: Tue, 19 Nov 2019 13:59:35 -0500
Subject: [PATCH] btrfs: do not call synchronize_srcu() in inode_tree_del

Testing with the new fsstress uncovered a pretty nasty deadlock with lookup and snapshot deletion.

Process A
unlink
-> final iput
-> inode_tree_del

     -> synchronize_srcu(subvol_srcu)

Process B
btrfs_lookup <- srcu_read_lock() acquired here
-> btrfs_iget

    -> find inode that has I_FREEING set
      -> __wait_on_freeing_inode()

We're holding the srcu_read_lock() while doing the iget in order to make sure our fs root doesn't go away, and then we are waiting for the inode to finish freeing. However because the free'ing process is doing a synchronize_srcu() we deadlock.

Fix this by dropping the synchronize_srcu() in inode_tree_del(). We don't need people to stop accessing the fs root at this point, we're only adding our empty root to the dead roots list.

A larger much more invasive fix is forthcoming to address how we deal with fs roots, but this fixes the immediate problem.

index 56032c518b26..5766c2d19896 100644
@@ -5728,7 +5728,6 @@ static void inode_tree_add(struct inode *inode)
 
 static void inode_tree_del(struct inode *inode)
 {
-	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	int empty = 0;
 

@@ -5741,7 +5740,6 @@ static void inode_tree_del(struct inode *inode)
 	spin_unlock(&root->inode_lock);
 
 	if (empty && btrfs_root_refs(&root->root_item) == 0) {
-		synchronize_srcu(&fs_info->subvol_srcu);
 		spin_lock(&root->inode_lock);
 		empty = RB_EMPTY_ROOT(&root->inode_tree);
 		spin_unlock(&root->inode_lock);