Re: [PATCH v8 3/8] writeback, cgroup: increment isw_nr_in_flight before grabbing an inode

Jan Kara
2021-06-08T08:45:44Z

On Mon 07-06-21 18:31:18, Roman Gushchin wrote:
> isw_nr_in_flight is used do determine whether the inode switch queue
> should be flushed from the umount path. Currently it's increased
> after grabbing an inode and even scheduling the switch work. It means
> the umount path can be walked past cleanup_offline_cgwb() with active
> inode references, which can result in a "Busy inodes after unmount."
> message and use-after-free issues (with inode->i_sb which gets freed).
>
> Fix it by incrementing isw_nr_in_flight before doing anything with
> the inode and decrementing in the case when switching wasn't scheduled.
>
> The problem hasn't yet been seen in the real life and was discovered
> by Jan Kara by looking into the code.
>
> Suggested-by: Jan Kara <jack-IBi9RG/b67k@public.gmane.org>
> Signed-off-by: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>

Honza

> ---
> fs/fs-writeback.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 3564efcc4b78..e2cc860a001b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -505,6 +505,8 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
> if (!isw)
> return;
>
> + atomic_inc(&isw_nr_in_flight);
> +
> /* find and pin the new wb */
> rcu_read_lock();
> memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
> @@ -535,11 +537,10 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
> * Let's continue after I_WB_SWITCH is guaranteed to be visible.
> */
> call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
> -
> - atomic_inc(&isw_nr_in_flight);
> return;
>
> out_free:
> + atomic_dec(&isw_nr_in_flight);
> if (isw->new_wb)
> wb_put(isw->new_wb);
> kfree(isw);
> --
> 2.31.1
>
--
Jan Kara <jack-IBi9RG/b67k@public.gmane.org>
SUSE Labs, CR