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    SubjectRe: [PATCH] Fix the race between smp_call_function and CPU booting
    On Tue, 2012-03-20 at 18:44 +0530, Srivatsa S. Bhat wrote:
    > I don't think this patch would change anything, atleast it wouldn't get
    > rid of the warning that Liu reported. Because, he is running his stress
    > tests on a machine which has only 2 CPUs. So effectively, we are hotplugging
    > only CPU1 (since CPU0 can't be taken offline, on Intel boxes).
    > Also, CPU1 is removed from cpu_active_mask during CPU_DOWN_PREPARE time itself,
    > and migrate_tasks() comes much later (during CPU_DYING). And in any case,
    > dest_cpu will never be CPU1, because it is the CPU that is going down. So it
    > *has* to be CPU0 anyway.
    > So, I don't think changes to select_fallback_rq() to make it more careful is
    > going to make any difference in the particular scenario that Liu is testing.
    > That said, even I don't know what the root cause of the warning is.. :-(

    Its a race in cpu-up, we set active before online, when we do a wakeup
    select_task_rq() will see !cpu_online(), we then call
    select_fallback_rq() to compute a new cpu, select_fallback_rq() computes
    a new cpu against cpu_active (which is set) and can thus return cpu 1,
    even though it is still offline.

    So we queue the task on cpu 1 and send a reschedule ipi, at which point
    we'll get the reported warning.

    My change modifies select_fallback_rq() to require online && active.

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