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
