From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Eric W. Biederman" Date: Wed, 29 Jun 2022 03:39:59 +0000 Subject: Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state Message-Id: <87pmisqgs0.fsf@email.froward.int.ebiederm.org> List-Id: References: <87a6bv6dl6.fsf_-_@email.froward.int.ebiederm.org> <20220505182645.497868-12-ebiederm@xmission.com> <877d5ajesi.fsf@email.froward.int.ebiederm.org> <87y1xk8zx5.fsf@email.froward.int.ebiederm.org> <87czess94h.fsf@email.froward.int.ebiederm.org> <20220628184850.05f60d1e@gandalf.local.home> In-Reply-To: <20220628184850.05f60d1e@gandalf.local.home> (Steven Rostedt's message of "Tue, 28 Jun 2022 18:48:50 -0400") MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Steven Rostedt Cc: Alexander Gordeev , linux-kernel@vger.kernel.org, rjw@rjwysocki.net, Oleg Nesterov , mingo@kernel.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, mgorman@suse.de, bigeasy@linutronix.de, Will Deacon , tj@kernel.org, linux-pm@vger.kernel.org, Peter Zijlstra , Richard Weinberger , Anton Ivanov , Johannes Berg , linux-um@lists.infradead.org, Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org, Kees Cook , Jann Horn , linux-ia64@vger.kernel.org Steven Rostedt writes: > On Tue, 28 Jun 2022 17:42:22 -0500 > "Eric W. Biederman" wrote: > >> diff --git a/kernel/ptrace.c b/kernel/ptrace.c >> index 156a99283b11..cb85bcf84640 100644 >> --- a/kernel/ptrace.c >> +++ b/kernel/ptrace.c >> @@ -202,6 +202,7 @@ static bool ptrace_freeze_traced(struct task_struct *task) >> spin_lock_irq(&task->sighand->siglock); >> if (task_is_traced(task) && !looks_like_a_spurious_pid(task) && >> !__fatal_signal_pending(task)) { >> + smp_rmb(); >> task->jobctl |= JOBCTL_PTRACE_FROZEN; >> ret = true; >> } >> diff --git a/kernel/signal.c b/kernel/signal.c >> index edb1dc9b00dc..bcd576e9de66 100644 >> --- a/kernel/signal.c >> +++ b/kernel/signal.c >> @@ -2233,6 +2233,7 @@ static int ptrace_stop(int exit_code, int why, unsigned long message, >> return exit_code; >> >> set_special_state(TASK_TRACED); >> + smp_wmb(); >> current->jobctl |= JOBCTL_TRACED; >> > > Are not these both done under the sighand->siglock spinlock? > > That is, the two paths should already be synchronized, and the memory > barriers will not help anything inside the locks. The locking should (and > must) handle all that. I would presume so to. However the READ_ONCE that is going astray does not look like it is honoring that. So perhaps there is a bug in the s390 spin_lock barriers? Perhaps there is a subtle detail in the barriers that spin locks provide that we are overlooking? I just know the observed behavior is: - reading tsk->jobctl and seeing JOBCTL_TRACED set. - reading tsk->__state and seeing TASK_RUNNING. So unless PREEMPT_RT is enabled on s390. It looks like there is a barrier problem. Alexander do you have PREEMPT_RT enabled on s390? I have been assuming you don't but I figure I should ask and make certain as PREEMPT_RT can cause this kind of failure. Eric