Re: FAILED: patch "[PATCH] btrfs: do not call synchronize_srcu() in inode_tree_del" failed to apply to 4.9-stable tree

[Sasha Levin <sashal@kernel.org>]

On Mon, Dec 23, 2019 at 12:11:47PM -0500, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 4.9-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 <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From f72ff01df9cf5db25c76674cac16605992d15467 Mon Sep 17 00:00:00 2001
>From: Josef Bacik <josef@toxicpanda.com>
>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.
>
>Fixes: 76dda93c6ae2 ("Btrfs: add snapshot/subvolume destroy ioctl")
>CC: stable@vger.kernel.org # 4.4+
>Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>Reviewed-by: David Sterba <dsterba@suse.com>
>Signed-off-by: David Sterba <dsterba@suse.com>

Fixed up to work around missing 0b246afa62b0 ("btrfs: root->fs_info
cleanup, add fs_info convenience variables") and queued for 4.9 and 4.4.

-- 
Thanks,
Sasha