1 Review Checklist for RCU Patches
4 This document contains a checklist for producing and reviewing patches
5 that make use of RCU. Violating any of the rules listed below will
6 result in the same sorts of problems that leaving out a locking primitive
7 would cause. This list is based on experiences reviewing such patches
8 over a rather long period of time, but improvements are always welcome!
10 0. Is RCU being applied to a read-mostly situation? If the data
11 structure is updated more than about 10% of the time, then
12 you should strongly consider some other approach, unless
13 detailed performance measurements show that RCU is nonetheless
14 the right tool for the job.
16 Another exception is where performance is not an issue, and RCU
17 provides a simpler implementation. An example of this situation
18 is the dynamic NMI code in the Linux 2.6 kernel, at least on
19 architectures where NMIs are rare.
21 Yet another exception is where the low real-time latency of RCU's
22 read-side primitives is critically important.
24 1. Does the update code have proper mutual exclusion?
26 RCU does allow -readers- to run (almost) naked, but -writers- must
27 still use some sort of mutual exclusion, such as:
30 b. atomic operations, or
31 c. restricting updates to a single task.
33 If you choose #b, be prepared to describe how you have handled
34 memory barriers on weakly ordered machines (pretty much all of
35 them -- even x86 allows reads to be reordered), and be prepared
36 to explain why this added complexity is worthwhile. If you
37 choose #c, be prepared to explain how this single task does not
38 become a major bottleneck on big multiprocessor machines (for
39 example, if the task is updating information relating to itself
40 that other tasks can read, there by definition can be no
43 2. Do the RCU read-side critical sections make proper use of
44 rcu_read_lock() and friends? These primitives are needed
45 to prevent grace periods from ending prematurely, which
46 could result in data being unceremoniously freed out from
47 under your read-side code, which can greatly increase the
48 actuarial risk of your kernel.
50 As a rough rule of thumb, any dereference of an RCU-protected
51 pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()
52 or by the appropriate update-side lock.
54 3. Does the update code tolerate concurrent accesses?
56 The whole point of RCU is to permit readers to run without
57 any locks or atomic operations. This means that readers will
58 be running while updates are in progress. There are a number
59 of ways to handle this concurrency, depending on the situation:
61 a. Use the RCU variants of the list and hlist update
62 primitives to add, remove, and replace elements on an
63 RCU-protected list. Alternatively, use the RCU-protected
64 trees that have been added to the Linux kernel.
66 This is almost always the best approach.
68 b. Proceed as in (a) above, but also maintain per-element
69 locks (that are acquired by both readers and writers)
70 that guard per-element state. Of course, fields that
71 the readers refrain from accessing can be guarded by the
74 This works quite well, also.
76 c. Make updates appear atomic to readers. For example,
77 pointer updates to properly aligned fields will appear
78 atomic, as will individual atomic primitives. Operations
79 performed under a lock and sequences of multiple atomic
80 primitives will -not- appear to be atomic.
82 This can work, but is starting to get a bit tricky.
84 d. Carefully order the updates and the reads so that
85 readers see valid data at all phases of the update.
86 This is often more difficult than it sounds, especially
87 given modern CPUs' tendency to reorder memory references.
88 One must usually liberally sprinkle memory barriers
89 (smp_wmb(), smp_rmb(), smp_mb()) through the code,
90 making it difficult to understand and to test.
92 It is usually better to group the changing data into
93 a separate structure, so that the change may be made
94 to appear atomic by updating a pointer to reference
95 a new structure containing updated values.
97 4. Weakly ordered CPUs pose special challenges. Almost all CPUs
98 are weakly ordered -- even i386 CPUs allow reads to be reordered.
99 RCU code must take all of the following measures to prevent
100 memory-corruption problems:
102 a. Readers must maintain proper ordering of their memory
103 accesses. The rcu_dereference() primitive ensures that
104 the CPU picks up the pointer before it picks up the data
105 that the pointer points to. This really is necessary
106 on Alpha CPUs. If you don't believe me, see:
108 http://www.openvms.compaq.com/wizard/wiz_2637.html
110 The rcu_dereference() primitive is also an excellent
111 documentation aid, letting the person reading the code
112 know exactly which pointers are protected by RCU.
114 The rcu_dereference() primitive is used by the various
115 "_rcu()" list-traversal primitives, such as the
116 list_for_each_entry_rcu(). Note that it is perfectly
117 legal (if redundant) for update-side code to use
118 rcu_dereference() and the "_rcu()" list-traversal
119 primitives. This is particularly useful in code
120 that is common to readers and updaters.
122 b. If the list macros are being used, the list_add_tail_rcu()
123 and list_add_rcu() primitives must be used in order
124 to prevent weakly ordered machines from misordering
125 structure initialization and pointer planting.
126 Similarly, if the hlist macros are being used, the
127 hlist_add_head_rcu() primitive is required.
129 c. If the list macros are being used, the list_del_rcu()
130 primitive must be used to keep list_del()'s pointer
131 poisoning from inflicting toxic effects on concurrent
132 readers. Similarly, if the hlist macros are being used,
133 the hlist_del_rcu() primitive is required.
135 The list_replace_rcu() primitive may be used to
136 replace an old structure with a new one in an
139 d. Updates must ensure that initialization of a given
140 structure happens before pointers to that structure are
141 publicized. Use the rcu_assign_pointer() primitive
142 when publicizing a pointer to a structure that can
143 be traversed by an RCU read-side critical section.
145 5. If call_rcu(), or a related primitive such as call_rcu_bh() or
146 call_rcu_sched(), is used, the callback function must be
147 written to be called from softirq context. In particular,
150 6. Since synchronize_rcu() can block, it cannot be called from
151 any sort of irq context. Ditto for synchronize_sched() and
154 7. If the updater uses call_rcu(), then the corresponding readers
155 must use rcu_read_lock() and rcu_read_unlock(). If the updater
156 uses call_rcu_bh(), then the corresponding readers must use
157 rcu_read_lock_bh() and rcu_read_unlock_bh(). If the updater
158 uses call_rcu_sched(), then the corresponding readers must
159 disable preemption. Mixing things up will result in confusion
162 One exception to this rule: rcu_read_lock() and rcu_read_unlock()
163 may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
164 in cases where local bottom halves are already known to be
165 disabled, for example, in irq or softirq context. Commenting
166 such cases is a must, of course! And the jury is still out on
167 whether the increased speed is worth it.
169 8. Although synchronize_rcu() is slower than is call_rcu(), it
170 usually results in simpler code. So, unless update performance
171 is critically important or the updaters cannot block,
172 synchronize_rcu() should be used in preference to call_rcu().
174 An especially important property of the synchronize_rcu()
175 primitive is that it automatically self-limits: if grace periods
176 are delayed for whatever reason, then the synchronize_rcu()
177 primitive will correspondingly delay updates. In contrast,
178 code using call_rcu() should explicitly limit update rate in
179 cases where grace periods are delayed, as failing to do so can
180 result in excessive realtime latencies or even OOM conditions.
182 Ways of gaining this self-limiting property when using call_rcu()
185 a. Keeping a count of the number of data-structure elements
186 used by the RCU-protected data structure, including those
187 waiting for a grace period to elapse. Enforce a limit
188 on this number, stalling updates as needed to allow
189 previously deferred frees to complete.
191 Alternatively, limit only the number awaiting deferred
192 free rather than the total number of elements.
194 b. Limiting update rate. For example, if updates occur only
195 once per hour, then no explicit rate limiting is required,
196 unless your system is already badly broken. The dcache
197 subsystem takes this approach -- updates are guarded
198 by a global lock, limiting their rate.
200 c. Trusted update -- if updates can only be done manually by
201 superuser or some other trusted user, then it might not
202 be necessary to automatically limit them. The theory
203 here is that superuser already has lots of ways to crash
206 d. Use call_rcu_bh() rather than call_rcu(), in order to take
207 advantage of call_rcu_bh()'s faster grace periods.
209 e. Periodically invoke synchronize_rcu(), permitting a limited
210 number of updates per grace period.
212 9. All RCU list-traversal primitives, which include
213 rcu_dereference(), list_for_each_entry_rcu(),
214 list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
215 must be either within an RCU read-side critical section or
216 must be protected by appropriate update-side locks. RCU
217 read-side critical sections are delimited by rcu_read_lock()
218 and rcu_read_unlock(), or by similar primitives such as
219 rcu_read_lock_bh() and rcu_read_unlock_bh().
221 The reason that it is permissible to use RCU list-traversal
222 primitives when the update-side lock is held is that doing so
223 can be quite helpful in reducing code bloat when common code is
224 shared between readers and updaters.
226 10. Conversely, if you are in an RCU read-side critical section,
227 and you don't hold the appropriate update-side lock, you -must-
228 use the "_rcu()" variants of the list macros. Failing to do so
229 will break Alpha and confuse people reading your code.
231 11. Note that synchronize_rcu() -only- guarantees to wait until
232 all currently executing rcu_read_lock()-protected RCU read-side
233 critical sections complete. It does -not- necessarily guarantee
234 that all currently running interrupts, NMIs, preempt_disable()
235 code, or idle loops will complete. Therefore, if you do not have
236 rcu_read_lock()-protected read-side critical sections, do -not-
237 use synchronize_rcu().
239 If you want to wait for some of these other things, you might
240 instead need to use synchronize_irq() or synchronize_sched().
242 12. Any lock acquired by an RCU callback must be acquired elsewhere
243 with irq disabled, e.g., via spin_lock_irqsave(). Failing to
244 disable irq on a given acquisition of that lock will result in
245 deadlock as soon as the RCU callback happens to interrupt that
246 acquisition's critical section.
248 13. RCU callbacks can be and are executed in parallel. In many cases,
249 the callback code simply wrappers around kfree(), so that this
250 is not an issue (or, more accurately, to the extent that it is
251 an issue, the memory-allocator locking handles it). However,
252 if the callbacks do manipulate a shared data structure, they
253 must use whatever locking or other synchronization is required
254 to safely access and/or modify that data structure.
256 RCU callbacks are -usually- executed on the same CPU that executed
257 the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
258 but are by -no- means guaranteed to be. For example, if a given
259 CPU goes offline while having an RCU callback pending, then that
260 RCU callback will execute on some surviving CPU. (If this was
261 not the case, a self-spawning RCU callback would prevent the
262 victim CPU from ever going offline.)
264 14. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())
265 may only be invoked from process context. Unlike other forms of
266 RCU, it -is- permissible to block in an SRCU read-side critical
267 section (demarked by srcu_read_lock() and srcu_read_unlock()),
268 hence the "SRCU": "sleepable RCU". Please note that if you
269 don't need to sleep in read-side critical sections, you should
270 be using RCU rather than SRCU, because RCU is almost always
271 faster and easier to use than is SRCU.
273 Also unlike other forms of RCU, explicit initialization
274 and cleanup is required via init_srcu_struct() and
275 cleanup_srcu_struct(). These are passed a "struct srcu_struct"
276 that defines the scope of a given SRCU domain. Once initialized,
277 the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
278 and synchronize_srcu(). A given synchronize_srcu() waits only
279 for SRCU read-side critical sections governed by srcu_read_lock()
280 and srcu_read_unlock() calls that have been passd the same
281 srcu_struct. This property is what makes sleeping read-side
282 critical sections tolerable -- a given subsystem delays only
283 its own updates, not those of other subsystems using SRCU.
284 Therefore, SRCU is less prone to OOM the system than RCU would
285 be if RCU's read-side critical sections were permitted to
288 The ability to sleep in read-side critical sections does not
289 come for free. First, corresponding srcu_read_lock() and
290 srcu_read_unlock() calls must be passed the same srcu_struct.
291 Second, grace-period-detection overhead is amortized only
292 over those updates sharing a given srcu_struct, rather than
293 being globally amortized as they are for other forms of RCU.
294 Therefore, SRCU should be used in preference to rw_semaphore
295 only in extremely read-intensive situations, or in situations
296 requiring SRCU's read-side deadlock immunity or low read-side
299 Note that, rcu_assign_pointer() and rcu_dereference() relate to
300 SRCU just as they do to other forms of RCU.
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