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    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


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