Re: [PATCH v1 1/2] Add polling support to pidfd

[Oleg Nesterov]

On 04/29, Joel Fernandes wrote:
>
> On Mon, Apr 29, 2019 at 04:20:30PM +0200, Oleg Nesterov wrote:
> > On 04/29, Joel Fernandes wrote:
> > >
> > > However, in your code above, it is avoided because we get:
> > >
> > > Task A (poller)		Task B (exiting task being polled)
> > > ------------            ----------------
> > > poll() called
> > > add_wait_queue()
> > > 			exit_state is set to non-zero
> > > read exit_state
> > > remove_wait_queue()
> > > 			wake_up_all()
> >
> > just to clarify... No, sys_poll() path doesn't do remove_wait_queue() until
> > it returns to user mode, and that is why we can't race with set-exit_code +
> > wake_up().
>
> I didn't follow what you mean, the removal from the waitqueue happens in
> free_poll_entry() called from poll_freewait() which happens from
> do_sys_poll() which is before the syscall returns to user mode. Could you
> explain more?

Hmm. I do not really understand the question... Sure, do_sys_poll() does
poll_freewait() before sysret or even before return from syscall, but why
does this matter? This is the exit path, it frees the memory, does fput(),
etc, f_op->poll() won't be call after that.

> > pidfd_poll() can race with the exiting task, miss exit_code != 0, and return
> > zero. However, do_poll() won't block after that and pidfd_poll() will be called
> > again.
>
> Here also I didn't follow what you mean. If exit_code is read as 0 in
> pidfd_poll(), then in do_poll() the count will be 0 and it will block in
> poll_schedule_timeout(). Right?

No. Please note the pwq->triggered check and please read __pollwake().

But if you want to understand this you can forget about poll/select. It is
a bit complicated, in particular because it has to do set_current_state()
right  before schedule() and thus it plays games with pwq->triggered. But in
essence this doesn't differ too much from the plain wait_event-like code
(although you can also look at wait_woken/woken_wake_function).

If remove_wait_queue() could happem before wake_up_all() (like in your pseudo-
code above), then pidfd_poll() or any other ->poll() method could miss _both_
the condition and wakeup. But sys_poll() doesn't do this, so it is fine to miss
the condition and rely on wake_up_all() which ensures we won't block and the
next iteration must see condition == T.

Oleg.