lkml.org 
[lkml]   [2010]   [Feb]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: rcu_dereference() without protection in select_task_rq_fair()
On Mon, Feb 15, 2010 at 10:12:06AM +0100, Peter Zijlstra wrote:
> On Sun, 2010-02-14 at 09:04 -0800, Paul E. McKenney wrote:
>
> > OK, but doesn't the "preempt_count() != 0" that is in the current version
> > of rcu_read_lock_sched_held() already cover this check?
>
> Hmm, yes it should.
>
> > In other words, I believe that I have located a usage of for_each_domain()
> > that violates the rule that it may only be called within preempt-disabled
> > sections.
>
> >From the trace:
>
> > [<ffffffff81033ec4>] select_task_rq_fair+0xc1/0x686
> > [<ffffffff810353f4>] wake_up_new_task+0x1e/0x13e
>
> Which reads like:
>
> void wake_up_new_task(...)
> {
> ...
>
> int cpu __maybe_unused = get_cpu();
>
> #ifdef CONFIG_SMP
> /*
> * Fork balancing, do it here and not earlier because:
> * - cpus_allowed can change in the fork path
> * - any previously selected cpu might disappear through hotplug
> *
> * We still have TASK_WAKING but PF_STARTING is gone now, meaning
> * ->cpus_allowed is stable, we have preemption disabled, meaning
> * cpu_online_mask is stable.
> */
> cpu = select_task_rq(p, SD_BALANCE_FORK, 0);
> set_task_cpu(p, cpu);
> #endif
>
> ...
>
> put_cpu()
> }
>
> I cannot see how we can get there without preemption disabled.

Interesting point. I have seen this but once. If it reproduces, I will
instrument the code path and see if I can track it down.

Thanx, Paul


\
 
 \ /
  Last update: 2010-02-15 18:39    [from the cache]
©2003-2011 Jasper Spaans