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Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion

On Wed, Sep 13, 2017 at 06:10:48PM -0700, Jaegeuk Kim wrote:

> Android triggers umount(2) by init process, which is definitely not a kernel
> thread. But, we've seen some kernel panics which say umount(2) was succeeded,
> but ext4 triggered a kernel panic due to EIO after then like below. I'm also
> not sure task_work_run() would be also safe enoughly. May I ask where I can
> find sys_umount() calls task_work_run()?

ret_{fast,slow}_syscall ->
	slow_work_pending ->
		do_work_pending() ->
			tracehook_notify_resume() ->

It's not sys_umount() (or any other sys_...()) - it's syscall dispatcher after
having called one of those and before returning to userland.  What is guaranteed
is that after successful task_work_add() the damn thing will be run in context
of originating process before it returns from syscall.  So any subsequent
syscalls from that process are guaranteed to happen after the work has run.
The same happens if the process exits rather than returns to userland (do_exit() ->
exit_task_work() -> task_work_run()), but for that you would need it to die in
umount(2) (e.g. get kill -9 delivered on the way out).

Please, check if you are seeing task_work_add() failure in there and if you do,
I would like to see a stack trace.  IOW, slap WARN_ON(1); right after
                        if (!task_work_add(task, &mnt->mnt_rcu, true))
and see what (if anything) gets printed.