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    SubjectRe: [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use of RCU by offline CPUs
    On Wed, Feb 01, 2012 at 11:41:56AM -0800, Paul E. McKenney wrote:
    > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
    >
    > Because newly offlined CPUs continue executing after completing the
    > CPU_DYING notifiers, they legitimately enter the scheduler and use
    > RCU while appearing to be offline. This calls for a more sophisticated
    > approach as follows:
    >
    > 1. RCU marks the CPU online during the CPU_UP_PREPARE phase.
    >
    > 2. RCU marks the CPU offline during the CPU_DEAD phase.
    >
    > 3. Diagnostics regarding use of read-side RCU by offline CPUs use
    > RCU's accounting rather than the cpu_online_map. (Note that
    > __call_rcu() still uses cpu_online_map to detect illegal
    > invocations within CPU_DYING notifiers.)
    >
    > 4. Offline CPUs are prevented from hanging the system by
    > force_quiescent_state(), which pays attention to cpu_online_map.
    > Some additional work (in a later commit) will be needed to
    > guarantee that force_quiescent_state() waits a full jiffy before
    > assuming that a CPU is offline, for example, when called from
    > idle entry.
    >
    > This approach avoids the false positives encountered when attempting to
    > use more exact classification of CPU online/offline state.

    Doesn't this fix need to happen *before* the earlier patches in this
    series that add splats for RCU usage while offline? Otherwise,
    bisection can hit those splats.

    - Josh Triplett


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