From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: [PATCH v1 1/2] Add polling support to pidfd Date: Mon, 29 Apr 2019 16:20:30 +0200 Message-ID: <20190429142030.GA17715@redhat.com> References: <20190425190010.46489-1-joel@joelfernandes.org> <20190425222359.sqhboc4x4daznr6r@brauner.io> <20190428162405.GA6757@redhat.com> <20190429140245.GB233442@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190429140245.GB233442@google.com> Sender: linux-kernel-owner@vger.kernel.org To: Joel Fernandes Cc: Christian Brauner , linux-kernel@vger.kernel.org, luto@amacapital.net, rostedt@goodmis.org, dancol@google.com, sspatil@google.com, jannh@google.com, surenb@google.com, timmurray@google.com, Jonathan Kowalski , torvalds@linux-foundation.org, kernel-team@android.com, Andrew Morton , Arnd Bergmann , "Eric W. Biederman" , Greg Kroah-Hartman , Ingo Molnar , Jann Horn , linux-kselftest@vger.kernel.org, Michal Hocko , "Peter Zijlstra (Intel)" , Serge Hallyn , Shuah Khan , Stephen Rothwell , Thomas Gleixner List-Id: linux-api@vger.kernel.org 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.