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SubjectRe: [patch] rt, hotplug: Use set_cpus_allowed_ptr() in sync_unplug_thread()
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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.

-Mike


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