[PATCH] btrfs: do not call synchronize_srcu() in inode_tree_del

[PATCH] btrfs: do not call synchronize_srcu() in inode_tree_del

[Josef Bacik]

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.

Fixes: 76dda93c6ae2 ("Btrfs: add snapshot/subvolume destroy ioctl")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/inode.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 8db7455fee38..fc0624fbe387 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -5729,7 +5729,6 @@ 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;
 
@@ -5742,7 +5741,6 @@ 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);
-- 
2.23.0

Re: [PATCH] btrfs: do not call synchronize_srcu() in inode_tree_del

[David Sterba]

On Tue, Nov 19, 2019 at 01:59:35PM -0500, Josef Bacik wrote:
> 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.
> 
> Fixes: 76dda93c6ae2 ("Btrfs: add snapshot/subvolume destroy ioctl")
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Added to misc-next, thanks.