From mboxrd@z Thu Jan  1 00:00:00 1970
From: oleg@redhat.com (Oleg Nesterov)
Date: Tue, 30 Apr 2019 14:07:59 +0200
Subject: [PATCH v1 1/2] Add polling support to pidfd
In-Reply-To: <20190430115332.GB23020@redhat.com>
References: <20190425190010.46489-1-joel@joelfernandes.org>
 <20190425222359.sqhboc4x4daznr6r@brauner.io>
 <20190428162405.GA6757@redhat.com>
 <20190429140245.GB233442@google.com>
 <20190429142030.GA17715@redhat.com>
 <20190429163259.GA201155@google.com>
 <20190430115332.GB23020@redhat.com>
Message-ID: <20190430120759.GC23020@redhat.com>
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Message-ID: <20190430120759._4kNQHQq4fZteFY2bD0u3FgouWF0TDt2_7l4Dey7--4@z>

On 04/30, Oleg Nesterov wrote:
>
> > > 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.

Oh, just in case... If it is not clear, of course I am talking about the case
when wake_up_call() was already called when we check the condition. Otherwise
everything is simple.

Oleg.