From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755799AbeEAOXJ (ORCPT ); Tue, 1 May 2018 10:23:09 -0400 Received: from merlin.infradead.org ([205.233.59.134]:45564 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755132AbeEAOXH (ORCPT ); Tue, 1 May 2018 10:23:07 -0400 Date: Tue, 1 May 2018 16:22:51 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: mingo@kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, will.deacon@arm.com, mpe@ellerman.id.au, bigeasy@linutronix.de, gkohli@codeaurora.org, neeraju@codeaurora.org Subject: Re: [PATCH 2/2] sched: Introduce set_special_state() Message-ID: <20180501142251.GH12217@hirez.programming.kicks-ass.net> References: <20180430141751.377491406@infradead.org> <20180430142235.900370484@infradead.org> <20180430164545.GA10951@redhat.com> <20180430194024.GA12217@hirez.programming.kicks-ass.net> <20180501095054.GD12235@hirez.programming.kicks-ass.net> <20180501135924.GA12696@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180501135924.GA12696@redhat.com> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 01, 2018 at 03:59:24PM +0200, Oleg Nesterov wrote: > On 05/01, Peter Zijlstra wrote: > > > > The only code I found that seems to care is ptrace_attach(), where we > > wait for JOBCTL_TRAPPING to get cleared. That same function has a > > comment about hiding the STOPPED -> RUNNING -> TRACED transition. So I'm > > assuming it needs to observe TRACED if it observes !TRAPPING. > > Yes, exactly. > > > But I don't think there's enough barriers on that end to guarantee this. > > Any ->state load after wait_on_bit() is, afact, free to have happened > > before the ->jobctl load. > > do_wait() does set_current_state() before it checks ->state or anything else. But how are ptrace_attach() and do_wait() related? I guess I'm missing something fairly fundamental here. Is it userspace doing these two system calls back-to-back or something? Is that not a little bit fragile, to rely on a barrier in do_wait() for this? Anyway, does the below look ok? --- --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1961,14 +1961,26 @@ static void ptrace_stop(int exit_code, i return; } + set_special_state(TASK_TRACED); + /* * We're committing to trapping. TRACED should be visible before * TRAPPING is cleared; otherwise, the tracer might fail do_wait(). * Also, transition to TRACED and updates to ->jobctl should be * atomic with respect to siglock and should be done after the arch * hook as siglock is released and regrabbed across it. + * + * TRACER TRACEE + * ptrace_attach() + * [L] wait_on_bit(JOBCTL_TRAPPING) [S] set_special_state(TRACED) + * do_wait() + * set_current_state() smp_wmb(); + * ptrace_do_wait() + * wait_task_stopped() + * task_stopped_code() + * [L] task_is_traced() [S] task_clear_jobctl_trapping(); */ - set_special_state(TASK_TRACED); + smp_wmb(); current->last_siginfo = info; current->exit_code = exit_code;