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

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Date:   Tue, 28 Jun 2022 19:15:41 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.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

On Tue, 21 Jun 2022 17:15:47 +0200
Alexander Gordeev <agordeev@...ux.ibm.com> wrote:

> So I assume (checked actually) the return 0 below from kernel/sched/core.c:
> wait_task_inactive() is where it bails out:
> 
> 3303                 while (task_running(rq, p)) {
> 3304                         if (match_state && unlikely(READ_ONCE(p->__state) != match_state))
> 3305                                 return 0;
> 3306                         cpu_relax();
> 3307                 }
> 
> Yet, the child task is always found in __TASK_TRACED state (as seen
> in crash dumps):
> 
> > 101447  11342  13      ce3a8100      RU   0.0   10040   4412  strace  
>   101450  101447   0      bb04b200      TR   0.0    2272   1136  kill_child
>   108261  101447   2      d0b10100      TR   0.0    2272    532  kill_child
> crash> task bb04b200 __state  
> PID: 101450  TASK: bb04b200          CPU: 0   COMMAND: "kill_child"
>   __state = 8,
> 
> crash> task d0b10100 __state  
> PID: 108261  TASK: d0b10100          CPU: 2   COMMAND: "kill_child"
>   __state = 8,

If you are using crash, can you enable all trace events?

Then you should be able to extract the ftrace ring buffer from crash using
the trace.so extend (https://github.com/fujitsu/crash-trace)

I guess it should still work with s390.

Then you can see the events that lead up to the crash.

-- Steve