Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state

Eric W. Biederman
2022-06-29T03:39:59Z

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