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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Alexander Gordeev <agordeev@linux.ibm.com>,
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Date:   Tue, 28 Jun 2022 22:39:59 -0500
In-Reply-To: <20220628184850.05f60d1e@gandalf.local.home> (Steven Rostedt's
        message of "Tue, 28 Jun 2022 18:48:50 -0400")
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Subject: Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED,
 TASK_STOPPED state
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Steven Rostedt <rostedt@goodmis.org> writes:

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