Re: [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use ofRCU by offline CPUs

From: Josh Triplett
Date: Thu Feb 02 2012 - 01:11:23 EST

On Wed, Feb 01, 2012 at 11:41:56AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>
> 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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