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Date: Tue, 05 Jul 2022 19:28:49 +0200
From: Sven Schnelle <svens@...ux.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
"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
Sven Schnelle <svens@...ux.ibm.com> writes:
> Steven Rostedt <rostedt@...dmis.org> writes:
>
>> 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.
I think there's a race in ptrace_check_attach(). It first calls
ptrace_freeze_task(), which checks whether JOBCTL_TRACED is set.
If it is (and a few other conditions match) it will set ret = 0.
Later outside of siglock and tasklist_lock it will call
wait_task_inactive, assuming the target is in TASK_TRACED, but it isn't.
ptrace_stop(), which runs on another CPU, does:
set_special_state(TASK_TRACED);
current->jobctl |= JOBCTL_TRACED;
which looks ok on first sight, but in this case JOBCTL is already set,
so the reading CPU will immediately move on to wait_task_inactive(),
before JOBCTL_TRACED is set. I don't know whether this is a valid
combination. I never looked into JOBCTL_* semantics, but i guess now
is a good time to do so. I added some debugging statements, and that
gives:
[ 86.218488] kill_chi-300545 2d.... 79990135us : ptrace_stop: state 8
[ 86.218492] kill_chi-300545 2d.... 79990136us : signal_generate: sig=17 errno=0 code=4 comm=strace pid=300542 grp=1 res=1
[ 86.218496] kill_chi-300545 2d.... 79990136us : sched_stat_runtime: comm=kill_child pid=300545 runtime=3058 [ns] vruntime=606165713178 [ns]
[ 86.218500] kill_chi-300545 2d.... 79990136us : sched_switch: prev_comm=kill_child prev_pid=300545 prev_prio=120 prev_state=t ==> next_comm=swapper/2 next_pid=0 next_prio=120
[ 86.218504] strace-300542 7..... 79990139us : sys_ptrace -> 0x50
[ 86.218508] strace-300542 7..... 79990139us : sys_write(fd: 2, buf: 2aa198f7ad0, count: 12)
[ 86.218512] strace-300542 7..... 79990140us : sys_write -> 0x12
[ 86.218515] <idle>-0 6dNh.. 79990140us : sched_wakeup: comm=kill_child pid=343805 prio=120 target_cpu=006
[ 86.218519] <idle>-0 6d.... 79990140us : sched_switch: prev_comm=swapper/6 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kill_child next_pid=343805 next_prio=120
[ 86.218524] strace-300542 7..... 79990140us : sys_write(fd: 2, buf: 2aa198f7ad0, count: 19)
[ 86.218527] strace-300542 7..... 79990141us : sys_write -> 0x19
[ 86.218531] kill_chi-343805 6..... 79990141us : sys_sched_yield -> 0xffffffffffffffda
[ 86.218535] strace-300542 7..... 79990141us : sys_ptrace(request: 18, pid: 53efd, addr: 0, data: 0)
[ 86.218539] kill_chi-343805 6d.... 79990141us : signal_deliver: sig=9 errno=0 code=0 sa_handler=0 sa_flags=0
[ 86.218543] strace-300542 7d.... 79990141us : ptrace_check_attach: task_is_traced: 1, fatal signal pending: 0
[ 86.218547] strace-300542 7..... 79990141us : ptrace_check_attach: child->pid = 343805, child->__flags=0
[ 86.218551] kill_chi-343805 6d.... 79990141us : ptrace_stop: JOBCTL_TRACED already set, state=0 <------ valid combination of flags?
[ 86.218554] kill_chi-343805 6d.... 79990141us : ptrace_stop: state 8
[ 86.218558] kill_chi-343805 6d.... 79990142us : signal_generate: sig=17 errno=0 code=4 comm=strace pid=300542 grp=1 res=1
[ 86.218562] kill_chi-343805 6d.... 79990142us : sched_stat_runtime: comm=kill_child pid=343805 runtime=2135 [ns] vruntime=556109013931 [ns]
[ 86.218566] strace-300542 7..... 79990142us : wait_task_inactive: NO MATCH: state 0, match_state 8, pid 343805
[ 86.218570] kill_chi-343805 6d.... 79990142us : sched_switch: prev_comm=kill_child prev_pid=343805 prev_prio=120 prev_state=t ==>next_comm=swapper/6 next_pid=0 next_prio=120
View attachment "ptrace-debug.patch" of type "text/x-diff" (2758 bytes)
I'll continue debugging tomorrow, but maybe this helps already.
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