Re: [PATCH v9 3/8] writeback, cgroup: increment isw_nr_in_flight before grabbing an inode

[Ming Lei]

On Tue, Jun 08, 2021 at 04:02:20PM -0700, 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@xxxxxxxx>
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> ---
>  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 b6fc13a4962d..4413e005c28c 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);

smp_mb() may be required for ordering the WRITE in 'atomic_inc(&isw_nr_in_flight)'
and the following READ on 'inode->i_sb->s_flags & SB_ACTIVE'. Otherwise,
cgroup_writeback_umount() may observe zero of 'isw_nr_in_flight' because of
re-order of the two OPs, then miss the flush_workqueue().

Also this barrier should serve as pair of the one added in cgroup_writeback_umount(),
so maybe this patch should be merged with 2/8.


Thanks, 
Ming