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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

* 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?
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?

>Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
>Cc: stable-rt@vger.kernel.org
>---
> kernel/cpu.c |    2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>--- a/kernel/cpu.c
>+++ b/kernel/cpu.c
>@@ -265,7 +265,7 @@ static int sync_unplug_thread(void *data
> 	 * we don't want any more work on this CPU.
> 	 */
> 	current->flags &= ~PF_NO_SETAFFINITY;
>-	do_set_cpus_allowed(current, cpu_present_mask);
>+	set_cpus_allowed_ptr(current, cpu_present_mask);
> 	migrate_me();
> 	return 0;
> }

Sebastian