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

[Mike Galbraith <umgwanakikbuti@gmail.com>]

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