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

300 APPENDIX F. ANSWERS TO QUICK QUIZZES1 idx = srcu_read_lock(&ssa);2 synchronize_srcu(&ssb);3 srcu_read_unlock(&ssa, idx);45 /* . . . */67 idx = srcu_read_lock(&ssb);8 synchronize_srcu(&ssa);9 srcu_read_unlock(&ssb, idx);Figure F.6: Multistage SRCU DeadlocksQuick Quiz 8.31:Why doesn’t list_del_rcu() poison both the nextand prev pointers?Answer:Poisoning the next pointer would interfere withconcurrent RCU readers, who must use this pointer.However, RCU readers are forbidden from using theprev pointer, so it may safely be poisoned.Quick Quiz 8.32:Normally, any pointer subject to rcu_dereference() must always be updated usingrcu_assign_pointer(). What is an exception tothis rule?Answer:One such exception is when a multi-elementlinked data structure is initialized as a unitwhile inaccessible to other CPUs, and then asingle rcu_assign_pointer() is used to planta global pointer to this data structure. Theinitialization-time pointer assignments need not usercu_assign_pointer(), though any such assignmentsthat happen after the structure is globallyvisible must use rcu_assign_pointer().However, unless this initialization code is on animpressively hot code-path, it is probably wise touse rcu_assign_pointer() anyway, even though itis in theory unnecessary. It is all too easy for a”minor”changetoinvalidateyourcherishedassumptionsabout the initialization happening privately.Quick Quiz 8.33:Are there any downsides to the fact that thesetraversal and update primitives can be used withany of the RCU API family members?Answer:It can sometimes be difficult for automated codecheckers such as “sparse” (or indeed for humanbeings) to work out which type of RCU read-sidecritical section a given RCU traversal primitivecorresponds to. For example, consider the codeshown in Figure F.7.1 rcu_read_lock();2 preempt_disable();3 p = rcu_dereference(global_pointer);45 /* . . . */67 preempt_enable();8 rcu_read_unlock();Figure F.7: Diverse RCU Read-Side NestingIs the rcu_dereference() primitive in an RCUClassic or an RCU Sched critical section? Whatwould you have to do to figure this out?Quick Quiz 8.34:Why wouldn’t any deadlock in the RCU implementationin Figure 8.27 also be a deadlock in any otherRCU implementation?Answer:Suppose the functions foo() and bar() in FigureF.8 are invoked concurrently from differentCPUs. Thenfoo()willacquiremy_lock()online3,while bar() will acquire rcu_gp_lock on line 13.When foo() advances to line 4, it will attempt toacquire rcu_gp_lock, which is held by bar(). Then1 void foo(void)2 {3 spin_lock(&my_lock);4 rcu_read_lock();5 do_something();6 rcu_read_unlock();7 do_something_else();8 spin_unlock(&my_lock);9 }1011 void bar(void)12 {13 rcu_read_lock();14 spin_lock(&my_lock);15 do_some_other_thing();16 spin_unlock(&my_lock);17 do_whatever();18 rcu_read_unlock();19 }Figure F.8: Deadlock in Lock-Based RCU Implementation