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

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Date:   Tue, 30 Apr 2019 14:07:59 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org, luto@...capital.net,
        rostedt@...dmis.org, dancol@...gle.com, sspatil@...gle.com,
        jannh@...gle.com, surenb@...gle.com, timmurray@...gle.com,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        torvalds@...ux-foundation.org, kernel-team@...roid.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>, Jann Horn <jann@...jh.net>,
        linux-kselftest@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Serge Hallyn <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, viro@...iv.linux.org.uk,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v1 1/2] Add polling support to pidfd

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.