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

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Message-ID: <20190429142030.GA17715@redhat.com>
Date:   Mon, 29 Apr 2019 16:20:30 +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 04/29, 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().

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.

Oleg.