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SubjectRe: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion
On Thu, Sep 14, 2017 at 02:30:17AM +0100, Al Viro wrote:
> 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() ->
> task_work_run()
>
> 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))
> return;
> and see what (if anything) gets printed.

AFAICS, for task_work_add() to fail here we need a final mntput() to be run
in context of a thread that already had exit_signals() run *and* subsequent
task_work_run() run to completion (with all pending callbacks executed, along
with all callbacks added by those, etc.)

For that to have happened during umount(2) we would've needed
* killing signal delivered while going through the syscall
* final mntput() to have been done *NOT* from sys_umount() (otherwise
the work would've been added before we got to exit_signals())
* final mntput() to have been done *NOT* from any task_work callbacks
(otherwise it would've been added before we'd observed a combination of empty
list of pending work with PF_EXITING)

I really want to see the stack trace of that failing task_work_add(), if that's
what actually happens there. What kind of a reproducer do you have for that?

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