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!
--
>From 282861286074c47907759d80c01419f0d0630dae Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@xxxxxx>
Date: Wed, 9 Jun 2021 14:14:26 -0700
Subject: [PATCH] cgroup, writeback: add smp_mb() to inode_prepare_wbs_switch()
Add a memory barrier between incrementing isw_nr_in_flight
and checking the sb's SB_ACTIVE flag and grabbing an inode in
inode_prepare_wbs_switch(). It's required to prevent grabbing
an inode before incrementing isw_nr_in_flight, otherwise
0 can be obtained as isw_nr_in_flight in cgroup_writeback_umount()
and isw_wq will not be flushed, potentially leading to a memory
corruption.
Added smp_mb() will work in pair with smp_mb() in
cgroup_writeback_umount().
Suggested-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Roman Gushchin <guro@xxxxxx>
---
fs/fs-writeback.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 545fce68e919..6332b86ca4ed 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -513,6 +513,14 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
static bool inode_prepare_wbs_switch(struct inode *inode,
struct bdi_writeback *new_wb)
{
+ /*
+ * Paired with smp_mb() in cgroup_writeback_umount().
+ * isw_nr_in_flight must be increased before checking SB_ACTIVE and
+ * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0
+ * in cgroup_writeback_umount() and the isw_wq will be not flushed.
+ */
+ smp_mb();
+
/* while holding I_WB_SWITCH, no one else can update the association */
spin_lock(&inode->i_lock);
if (!(inode->i_sb->s_flags & SB_ACTIVE) ||
--
2.31.1
