| Date | Thu, 2 Feb 2012 10:31:22 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use of RCU by offline CPUs |
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On Wed, Feb 01, 2012 at 10:11:06PM -0800, Josh Triplett wrote: > 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.
If someone actually does hit the splats during a real bisection, I will buy you the beverage of your choice. ;-)
Thanx, Paul
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