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Date:   Mon, 29 Apr 2019 16:20:30 +0200
From:   Oleg Nesterov <>
To:     Joel Fernandes <>
Cc:     Christian Brauner <>,,,,,,,,,
        Jonathan Kowalski <>,,,
        Andrew Morton <>,
        Arnd Bergmann <>,
        "Eric W. Biederman" <>,
        Greg Kroah-Hartman <>,
        Ingo Molnar <>, Jann Horn <>,, Michal Hocko <>,
        "Peter Zijlstra (Intel)" <>,
        Serge Hallyn <>, Shuah Khan <>,
        Stephen Rothwell <>,
        Thomas Gleixner <>,
        Tycho Andersen <>,,
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 +

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


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