From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joel Fernandes Subject: Re: [PATCH v1 1/2] Add polling support to pidfd Date: Mon, 29 Apr 2019 12:32:59 -0400 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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190429142030.GA17715@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Oleg Nesterov 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 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