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

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Thu, 2015-04-09 at 16:54 +0200, Sebastian Andrzej Siewior wrote:
> 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?

[exception RIP: set_cpus_allowed_rt+18] is BUG_ON(!rt_task(p)).

        -Mike