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

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Apr 30, 2019 at 01:53:33PM +0200, Oleg Nesterov wrote:
> 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.

Ok, we are on the same page on this.

> > > 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.

Agreed. In my pseudo-code above, I meant removal from waitqueue only once we
are not going to be blocking in poll and returning to userspace. I may have
messed the sequence of events, but my point was to show the race I had in
mind (missing a wake up due to adding to the waitqueue too late inside
pidfd_poll()).  Anyway, I will repost with your suggested change and send it
soon. Thanks for the discussions.

thanks,

 - Joel