The patch titled
Subject: fs/writeback.c: use rcu_barrier() to wait for inflight wb switches going into workqueue when umount
has been added to the -mm tree. Its filename is
fs-writeback-use-rcu_barrier-to-wait-for-inflight-wb-switches-going-into-workqueue-when-umount.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/fs-writeback-use-rcu_barrier-to-wait-for-inflight-wb-switches-going-into-workqueue-when-umount.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/fs-writeback-use-rcu_barrier-to-wait-for-inflight-wb-switches-going-into-workqueue-when-umount.patch
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------------------------------------------------------
From: Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx>
Subject: fs/writeback.c: use rcu_barrier() to wait for inflight wb switches going into workqueue when umount
synchronize_rcu() didn't wait for call_rcu() callbacks, so inode wb switch
may not go to the workqueue after synchronize_rcu(). Thus previous
scheduled switches was not finished even flushing the workqueue, which
will cause a NULL pointer dereferenced followed below.
VFS: Busy inodes after unmount of vdd. Self-destruct in 5 seconds. Have a nice day...
BUG: unable to handle kernel NULL pointer dereference at 0000000000000278
[<ffffffff8126a303>] evict+0xb3/0x180
[<ffffffff8126a760>] iput+0x1b0/0x230
[<ffffffff8127c690>] inode_switch_wbs_work_fn+0x3c0/0x6a0
[<ffffffff810a5b2e>] worker_thread+0x4e/0x490
[<ffffffff810a5ae0>] ? process_one_work+0x410/0x410
[<ffffffff810ac056>] kthread+0xe6/0x100
[<ffffffff8173c199>] ret_from_fork+0x39/0x50
Replace the synchronize_rcu() call with a rcu_barrier() to wait for all
pending callbacks to finish. And inc isw_nr_in_flight after call_rcu() in
inode_switch_wbs() to make more sense.
Link: http://lkml.kernel.org/r/20190429024108.54150-1-jiufei.xue@xxxxxxxxxxxxxxxxx
Signed-off-by: Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/fs-writeback.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
--- a/fs/fs-writeback.c~fs-writeback-use-rcu_barrier-to-wait-for-inflight-wb-switches-going-into-workqueue-when-umount
+++ a/fs/fs-writeback.c
@@ -523,8 +523,6 @@ static void inode_switch_wbs(struct inod
isw->inode = inode;
- atomic_inc(&isw_nr_in_flight);
-
/*
* In addition to synchronizing among switchers, I_WB_SWITCH tells
* the RCU protected stat update paths to grab the i_page
@@ -532,6 +530,9 @@ static void inode_switch_wbs(struct inod
* 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);
+
goto out_unlock;
out_free:
@@ -901,7 +902,11 @@ restart:
void cgroup_writeback_umount(void)
{
if (atomic_read(&isw_nr_in_flight)) {
- synchronize_rcu();
+ /*
+ * Use rcu_barrier() to wait for all pending callbacks to
+ * ensure that all in-flight wb switches are in the workqueue.
+ */
+ rcu_barrier();
flush_workqueue(isw_wq);
}
}
_
Patches currently in -mm which might be from jiufei.xue@xxxxxxxxxxxxxxxxx are
fs-writeback-use-rcu_barrier-to-wait-for-inflight-wb-switches-going-into-workqueue-when-umount.patch
