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Date:   Mon, 29 Apr 2019 12:32:59 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Christian Brauner <christian@brauner.io>,
        linux-kernel@vger.kernel.org, luto@amacapital.net,
        rostedt@goodmis.org, dancol@google.com, sspatil@google.com,
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        Jonathan Kowalski <bl0pbl33p@gmail.com>,
        torvalds@linux-foundation.org, kernel-team@android.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>, Jann Horn <jann@thejh.net>,
        linux-kselftest@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Serge Hallyn <serge@hallyn.com>, Shuah Khan <shuah@kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tycho Andersen <tycho@tycho.ws>, viro@zeniv.linux.org.uk,
        linux-api@vger.kernel.org
Subject: Re: [PATCH v1 1/2] Add polling support to pidfd
Message-ID: <20190429163259.GA201155@google.com>
References: <20190425190010.46489-1-joel@joelfernandes.org>
 <20190425222359.sqhboc4x4daznr6r@brauner.io>
 <20190428162405.GA6757@redhat.com>
 <20190429140245.GB233442@google.com>
 <20190429142030.GA17715@redhat.com>
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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?

> 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? But above you're saying it wont block.
Also if you could show a timing diagram of this different race you're talking
about, that will make things clear. It is a bit hard for me to picture
otherwise.

Also, I will use task_pid() for getting the pid from the task, as you suggest
in the other thread.

thanks,

- Joel