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SubjectRe: [patch] rt, hotplug: Use set_cpus_allowed_ptr() in sync_unplug_thread()
On 04/09/2015 04:23 PM, Mike Galbraith wrote:
> On Thu, 2015-04-09 at 16:05 +0200, Sebastian Andrzej Siewior wrote:
>> * Mike Galbraith | 2015-03-24 08:14:49 [+0100]:
>>
>>> do_set_cpus_allowed() is not safe vs ->sched_class change.
>>>
>>> crash> bt
>>> PID: 11676 TASK: ffff88026f979da0 CPU: 22 COMMAND:
>>> "sync_unplug/22"
>>> #0 [ffff880274d25bc8] machine_kexec at ffffffff8103b41c
>>> #1 [ffff880274d25c18] crash_kexec at ffffffff810d881a
>>> #2 [ffff880274d25cd8] oops_end at ffffffff81525818
>>> #3 [ffff880274d25cf8] do_invalid_op at ffffffff81003096
>>> #4 [ffff880274d25d90] invalid_op at ffffffff8152d3de
>>> [exception RIP: set_cpus_allowed_rt+18]
>>> RIP: ffffffff8109e012 RSP: ffff880274d25e48 RFLAGS: 00010202
>>> RAX: ffffffff8109e000 RBX: ffff88026f979da0 RCX:
>>> ffff8802770cb6e8
>>> RDX: 0000000000000000 RSI: ffffffff81add700 RDI:
>>> ffff88026f979da0
>>> RBP: ffff880274d25e78 R8: ffffffff816112e0 R9:
>>> 0000000000000001
>>> R10: 0000000000000001 R11: 0000000000011940 R12:
>>> ffff88026f979da0
>>> R13: ffff8802770cb6d0 R14: ffff880274d25fd8 R15:
>>> 0000000000000000
>>> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
>>> #5 [ffff880274d25e60] do_set_cpus_allowed at ffffffff8108e65f
>>> #6 [ffff880274d25e80] sync_unplug_thread at ffffffff81058c08
>>> #7 [ffff880274d25ed8] kthread at ffffffff8107cad6
>>> #8 [ffff880274d25f50] ret_from_fork at ffffffff8152bbbc
>>> crash> task_struct ffff88026f979da0 | grep class
>>> sched_class = 0xffffffff816111e0 <fair_sched_class+64>,
>>
>> Is this a one-time thing or can you reproduce this?
>
> Well, I can't reproduce it now, having fixed it ;-) Dunno how
> repeatable it would be if I un-fixed it.
>
>> What happen here? I doubt p vanished. +18 is mostlikely the
>> "migrate_disabled_updated()" check.
>>
>> I doubt p->sched_class->set_cpus_allowed or p->sched_class vanish
>> between testing for it and invoking it, or did it?
>
> Class changed under us. We saw rt task, called rt method, rt method
> said BUG_ON(!rt_task(p)), as task had become fair class.

but why does backtrace then end in do_set_cpus_allowed and not in
set_cpus_allowed_rt()? Is it possible to provide a backtrace which ends
in the BUG() statement in set_cpus_allowed_rt() if this is where it is
coming from?

> -Mike
Sebastian



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