On Thu, 18 Apr 2019 10:04:26 +0800 Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx> wrote:
> 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.
>
> ...
>
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
>
> ...
>
> @@ -901,7 +902,7 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
> void cgroup_writeback_umount(void)
> {
> if (atomic_read(&isw_nr_in_flight)) {
> - synchronize_rcu();
> + rcu_barrier();
> flush_workqueue(isw_wq);
> }
> }
it would be nice to have a comment here explaining why the barrier is
being performed.
