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

[Roman Gushchin]

On Wed, Jun 09, 2021 at 11:32:44AM +0800, Ming Lei wrote:
> 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.

Hi Ming!

Good point, I agree. How about a patch below?

Thanks!

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