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+Review Checklist for RCU Patches
+This document contains a checklist for producing and reviewing patches
+that make use of RCU. Violating any of the rules listed below will
+result in the same sorts of problems that leaving out a locking primitive
+would cause. This list is based on experiences reviewing such patches
+over a rather long period of time, but improvements are always welcome!
+0. Is RCU being applied to a read-mostly situation? If the data
+ structure is updated more than about 10% of the time, then
+ you should strongly consider some other approach, unless
+ detailed performance measurements show that RCU is nonetheless
+ the right tool for the job.
+ The other exception would be where performance is not an issue,
+ and RCU provides a simpler implementation. An example of this
+ situation is the dynamic NMI code in the Linux 2.6 kernel,
+ at least on architectures where NMIs are rare.
+1. Does the update code have proper mutual exclusion?
+ RCU does allow -readers- to run (almost) naked, but -writers- must
+ still use some sort of mutual exclusion, such as:
+ a. locking,
+ b. atomic operations, or
+ c. restricting updates to a single task.
+ If you choose #b, be prepared to describe how you have handled
+ memory barriers on weakly ordered machines (pretty much all of
+ them -- even x86 allows reads to be reordered), and be prepared
+ to explain why this added complexity is worthwhile. If you
+ choose #c, be prepared to explain how this single task does not
+ become a major bottleneck on big multiprocessor machines.
+2. Do the RCU read-side critical sections make proper use of
+ rcu_read_lock() and friends? These primitives are needed
+ to suppress preemption (or bottom halves, in the case of
+ rcu_read_lock_bh()) in the read-side critical sections,
+ and are also an excellent aid to readability.
+3. Does the update code tolerate concurrent accesses?
+ The whole point of RCU is to permit readers to run without
+ any locks or atomic operations. This means that readers will
+ be running while updates are in progress. There are a number
+ of ways to handle this concurrency, depending on the situation:
+ a. Make updates appear atomic to readers. For example,
+ pointer updates to properly aligned fields will appear
+ atomic, as will individual atomic primitives. Operations
+ performed under a lock and sequences of multiple atomic
+ primitives will -not- appear to be atomic.
+ This is almost always the best approach.
+ b. Carefully order the updates and the reads so that
+ readers see valid data at all phases of the update.
+ This is often more difficult than it sounds, especially
+ given modern CPUs' tendency to reorder memory references.
+ One must usually liberally sprinkle memory barriers
+ (smp_wmb(), smp_rmb(), smp_mb()) through the code,
+ making it difficult to understand and to test.
+ It is usually better to group the changing data into
+ a separate structure, so that the change may be made
+ to appear atomic by updating a pointer to reference
+ a new structure containing updated values.
+4. Weakly ordered CPUs pose special challenges. Almost all CPUs
+ are weakly ordered -- even i386 CPUs allow reads to be reordered.
+ RCU code must take all of the following measures to prevent
+ memory-corruption problems:
+ a. Readers must maintain proper ordering of their memory
+ accesses. The rcu_dereference() primitive ensures that
+ the CPU picks up the pointer before it picks up the data
+ that the pointer points to. This really is necessary
+ on Alpha CPUs. If you don't believe me, see:
+ http://www.openvms.compaq.com/wizard/wiz_2637.html
+ The rcu_dereference() primitive is also an excellent
+ documentation aid, letting the person reading the code
+ know exactly which pointers are protected by RCU.
+ The rcu_dereference() primitive is used by the various
+ "_rcu()" list-traversal primitives, such as the
+ list_for_each_entry_rcu().
+ b. If the list macros are being used, the list_del_rcu(),
+ list_add_tail_rcu(), and list_del_rcu() primitives must
+ be used in order to prevent weakly ordered machines from
+ misordering structure initialization and pointer planting.
+ Similarly, if the hlist macros are being used, the
+ hlist_del_rcu() and hlist_add_head_rcu() primitives
+ are required.
+ c. Updates must ensure that initialization of a given
+ structure happens before pointers to that structure are
+ publicized. Use the rcu_assign_pointer() primitive
+ when publicizing a pointer to a structure that can
+ be traversed by an RCU read-side critical section.
+ [The rcu_assign_pointer() primitive is in process.]
+5. If call_rcu(), or a related primitive such as call_rcu_bh(),
+ is used, the callback function must be written to be called
+ from softirq context. In particular, it cannot block.
+6. Since synchronize_kernel() blocks, it cannot be called from
+ any sort of irq context.
+7. If the updater uses call_rcu(), then the corresponding readers
+ must use rcu_read_lock() and rcu_read_unlock(). If the updater
+ uses call_rcu_bh(), then the corresponding readers must use
+ rcu_read_lock_bh() and rcu_read_unlock_bh(). Mixing things up
+ will result in confusion and broken kernels.
+ One exception to this rule: rcu_read_lock() and rcu_read_unlock()
+ may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
+ in cases where local bottom halves are already known to be
+ disabled, for example, in irq or softirq context. Commenting
+ such cases is a must, of course! And the jury is still out on
+ whether the increased speed is worth it.
+8. Although synchronize_kernel() is a bit slower than is call_rcu(),
+ it usually results in simpler code. So, unless update performance
+ is important or the updaters cannot block, synchronize_kernel()
+ should be used in preference to call_rcu().
+9. All RCU list-traversal primitives, which include
+ list_for_each_rcu(), list_for_each_entry_rcu(),
+ list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
+ must be within an RCU read-side critical section. RCU
+ read-side critical sections are delimited by rcu_read_lock()
+ and rcu_read_unlock(), or by similar primitives such as
+ rcu_read_lock_bh() and rcu_read_unlock_bh().
+ Use of the _rcu() list-traversal primitives outside of an
+ RCU read-side critical section causes no harm other than
+ a slight performance degradation on Alpha CPUs and some
+ confusion on the part of people trying to read the code.
+ Another way of thinking of this is "If you are holding the
+ lock that prevents the data structure from changing, why do
+ you also need RCU-based protection?" That said, there may
+ well be situations where use of the _rcu() list-traversal
+ primitives while the update-side lock is held results in
+ simpler and more maintainable code. The jury is still out
+ on this question.
+10. Conversely, if you are in an RCU read-side critical section,
+ you -must- use the "_rcu()" variants of the list macros.
+ Failing to do so will break Alpha and confuse people reading
+ your code.