Is Parallel Programming Hard, And, If So, What Can You Do About It?

318 APPENDIX F. ANSWERS TO QUICK QUIZZESis waiting on a quiescent state from an offline CPU.Quick Quiz D.26:So what guards the earlier fields in this structure?__get_cpu_var(rcu_bh_data)). Using the->rda[] array of whichever rcu_state structurewe were passed works correctly regardless of whichAPI __call_rcu() was invoked from (suggested byLai Jiangshan [Jia08]).Answer:Nothing does, as they are constants set at compiletime or boot time. Of course, the fields internal toeach rcu_node in the ->node array may change,but they are guarded separately.Quick Quiz D.27:I thought that RCU read-side processing wassupposed to be fast!!! The functions shown inFigure D.21 have so much junk in them that theyjust have to be slow!!! What gives here?Answer:Appearances can be deceiving. Thepreempt_disable(), preempt_enable(),local_bh_disable(), and local_bh_enable()each do a single non-atomic manipulation of localdata. Even that assumes CONFIG_PREEMPT,otherwise, the preempt_disable() andpreempt_enable() functions emit no code, noteven compiler directives. The __acquire() and__release() functions emit no code (not even compilerdirectives), but are instead used by the sparsesemantic-parsing bug-finding program. Finally,rcu_read_acquire() and rcu_read_release()emit no code (not even compiler directives) unlessthe “lockdep” lock-order debugging facility is enabled,in which case they can indeed be somewhatexpensive.In short, unless you are a kernel hacker who hasenabled debugging options, these functions are extremelycheap, and in some cases, absolutely freeof overhead. And, in the words of a Portland-areafurniture retailer, “free is a very good price”.Quick Quiz D.28:Why not simply use __get_cpu_var() to pick up areference to the current CPU’s rcu_data structureon line 13 in Figure D.22?Answer:Because we might be called either fromcall_rcu() (in which case we wouldneed __get_cpu_var(rcu_data)) or fromcall_rcu_bh() (in which case we would needQuick Quiz D.29:Given that rcu_pending() is always called twice onlines 29-32 of Figure D.23, shouldn’t there be someway to combine the checks of the two structures?Answer:Sorry, but this was a trick question. The Clanguage’s short-circuit boolean expression evaluationmeans that __rcu_pending() is invokedon rcu_bh_state only if the prior invocation onrcu_state returns zero.The reason the two calls are in this order is that“rcu” is used more heavily than is “rcu bh”, so thefirst call is more likely to return non-zero than is thesecond.Quick Quiz D.30:Shouldn’t line 42 of Figure D.23 also check forin_hardirq()?Answer:No. The rcu_read_lock_bh() primitive disablessoftirq, not hardirq. Because call_rcu_bh()need only wait for pre-existing “rcu bh” read-sidecritical sections to complete, we need only checkin_softirq().Quick Quiz D.31:Butdon’twealsoneedtocheckthatagraceperiodisactually in progress in __rcu_process_callbacksin Figure D.24?Answer:Indeed we do! And the first thing thatforce_quiescent_state() does is to performexactly that check.Quick Quiz D.32:What happens if two CPUs attempt to start a newgrace period concurrently in Figure D.24?Answer:One of the CPUs will be the first to acquire the rootrcu_node structure’s lock, and that CPU will start