linux-kernel - Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED, TASK

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Date:   Tue, 28 Jun 2022 22:39:59 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        Oleg Nesterov <oleg@...hat.com>, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED,
 TASK_STOPPED state

Steven Rostedt <rostedt@...dmis.org> writes:

> On Tue, 28 Jun 2022 17:42:22 -0500
> "Eric W. Biederman" <ebiederm@...ssion.com> 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