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1 | Review Checklist for RCU Patches |
2 | ||
3 | ||
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! | |
9 | ||
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. | |
15 | ||
16 | The other exception would be where performance is not an issue, | |
17 | and RCU provides a simpler implementation. An example of this | |
18 | situation is the dynamic NMI code in the Linux 2.6 kernel, | |
19 | at least on architectures where NMIs are rare. | |
20 | ||
21 | 1. Does the update code have proper mutual exclusion? | |
22 | ||
23 | RCU does allow -readers- to run (almost) naked, but -writers- must | |
24 | still use some sort of mutual exclusion, such as: | |
25 | ||
26 | a. locking, | |
27 | b. atomic operations, or | |
28 | c. restricting updates to a single task. | |
29 | ||
30 | If you choose #b, be prepared to describe how you have handled | |
31 | memory barriers on weakly ordered machines (pretty much all of | |
32 | them -- even x86 allows reads to be reordered), and be prepared | |
33 | to explain why this added complexity is worthwhile. If you | |
34 | choose #c, be prepared to explain how this single task does not | |
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35 | become a major bottleneck on big multiprocessor machines (for |
36 | example, if the task is updating information relating to itself | |
37 | that other tasks can read, there by definition can be no | |
38 | bottleneck). | |
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39 | |
40 | 2. Do the RCU read-side critical sections make proper use of | |
41 | rcu_read_lock() and friends? These primitives are needed | |
42 | to suppress preemption (or bottom halves, in the case of | |
43 | rcu_read_lock_bh()) in the read-side critical sections, | |
44 | and are also an excellent aid to readability. | |
45 | ||
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46 | As a rough rule of thumb, any dereference of an RCU-protected |
47 | pointer must be covered by rcu_read_lock() or rcu_read_lock_bh() | |
48 | or by the appropriate update-side lock. | |
49 | ||
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50 | 3. Does the update code tolerate concurrent accesses? |
51 | ||
52 | The whole point of RCU is to permit readers to run without | |
53 | any locks or atomic operations. This means that readers will | |
54 | be running while updates are in progress. There are a number | |
55 | of ways to handle this concurrency, depending on the situation: | |
56 | ||
57 | a. Make updates appear atomic to readers. For example, | |
58 | pointer updates to properly aligned fields will appear | |
59 | atomic, as will individual atomic primitives. Operations | |
60 | performed under a lock and sequences of multiple atomic | |
61 | primitives will -not- appear to be atomic. | |
62 | ||
63 | This is almost always the best approach. | |
64 | ||
65 | b. Carefully order the updates and the reads so that | |
66 | readers see valid data at all phases of the update. | |
67 | This is often more difficult than it sounds, especially | |
68 | given modern CPUs' tendency to reorder memory references. | |
69 | One must usually liberally sprinkle memory barriers | |
70 | (smp_wmb(), smp_rmb(), smp_mb()) through the code, | |
71 | making it difficult to understand and to test. | |
72 | ||
73 | It is usually better to group the changing data into | |
74 | a separate structure, so that the change may be made | |
75 | to appear atomic by updating a pointer to reference | |
76 | a new structure containing updated values. | |
77 | ||
78 | 4. Weakly ordered CPUs pose special challenges. Almost all CPUs | |
79 | are weakly ordered -- even i386 CPUs allow reads to be reordered. | |
80 | RCU code must take all of the following measures to prevent | |
81 | memory-corruption problems: | |
82 | ||
83 | a. Readers must maintain proper ordering of their memory | |
84 | accesses. The rcu_dereference() primitive ensures that | |
85 | the CPU picks up the pointer before it picks up the data | |
86 | that the pointer points to. This really is necessary | |
87 | on Alpha CPUs. If you don't believe me, see: | |
88 | ||
89 | http://www.openvms.compaq.com/wizard/wiz_2637.html | |
90 | ||
91 | The rcu_dereference() primitive is also an excellent | |
92 | documentation aid, letting the person reading the code | |
93 | know exactly which pointers are protected by RCU. | |
94 | ||
95 | The rcu_dereference() primitive is used by the various | |
96 | "_rcu()" list-traversal primitives, such as the | |
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97 | list_for_each_entry_rcu(). Note that it is perfectly |
98 | legal (if redundant) for update-side code to use | |
99 | rcu_dereference() and the "_rcu()" list-traversal | |
100 | primitives. This is particularly useful in code | |
101 | that is common to readers and updaters. | |
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103 | b. If the list macros are being used, the list_add_tail_rcu() |
104 | and list_add_rcu() primitives must be used in order | |
105 | to prevent weakly ordered machines from misordering | |
106 | structure initialization and pointer planting. | |
1da177e4 | 107 | Similarly, if the hlist macros are being used, the |
a83f1fe2 | 108 | hlist_add_head_rcu() primitive is required. |
1da177e4 | 109 | |
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110 | c. If the list macros are being used, the list_del_rcu() |
111 | primitive must be used to keep list_del()'s pointer | |
112 | poisoning from inflicting toxic effects on concurrent | |
113 | readers. Similarly, if the hlist macros are being used, | |
114 | the hlist_del_rcu() primitive is required. | |
115 | ||
116 | The list_replace_rcu() primitive may be used to | |
117 | replace an old structure with a new one in an | |
118 | RCU-protected list. | |
119 | ||
120 | d. Updates must ensure that initialization of a given | |
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121 | structure happens before pointers to that structure are |
122 | publicized. Use the rcu_assign_pointer() primitive | |
123 | when publicizing a pointer to a structure that can | |
124 | be traversed by an RCU read-side critical section. | |
125 | ||
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126 | 5. If call_rcu(), or a related primitive such as call_rcu_bh(), |
127 | is used, the callback function must be written to be called | |
128 | from softirq context. In particular, it cannot block. | |
129 | ||
a83f1fe2 | 130 | 6. Since synchronize_rcu() can block, it cannot be called from |
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131 | any sort of irq context. |
132 | ||
133 | 7. If the updater uses call_rcu(), then the corresponding readers | |
134 | must use rcu_read_lock() and rcu_read_unlock(). If the updater | |
135 | uses call_rcu_bh(), then the corresponding readers must use | |
136 | rcu_read_lock_bh() and rcu_read_unlock_bh(). Mixing things up | |
137 | will result in confusion and broken kernels. | |
138 | ||
139 | One exception to this rule: rcu_read_lock() and rcu_read_unlock() | |
140 | may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh() | |
141 | in cases where local bottom halves are already known to be | |
142 | disabled, for example, in irq or softirq context. Commenting | |
143 | such cases is a must, of course! And the jury is still out on | |
144 | whether the increased speed is worth it. | |
145 | ||
a83f1fe2 | 146 | 8. Although synchronize_rcu() is a bit slower than is call_rcu(), |
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147 | it usually results in simpler code. So, unless update |
148 | performance is critically important or the updaters cannot block, | |
149 | synchronize_rcu() should be used in preference to call_rcu(). | |
150 | ||
151 | An especially important property of the synchronize_rcu() | |
152 | primitive is that it automatically self-limits: if grace periods | |
153 | are delayed for whatever reason, then the synchronize_rcu() | |
154 | primitive will correspondingly delay updates. In contrast, | |
155 | code using call_rcu() should explicitly limit update rate in | |
156 | cases where grace periods are delayed, as failing to do so can | |
157 | result in excessive realtime latencies or even OOM conditions. | |
158 | ||
159 | Ways of gaining this self-limiting property when using call_rcu() | |
160 | include: | |
161 | ||
162 | a. Keeping a count of the number of data-structure elements | |
163 | used by the RCU-protected data structure, including those | |
164 | waiting for a grace period to elapse. Enforce a limit | |
165 | on this number, stalling updates as needed to allow | |
166 | previously deferred frees to complete. | |
167 | ||
168 | Alternatively, limit only the number awaiting deferred | |
169 | free rather than the total number of elements. | |
170 | ||
171 | b. Limiting update rate. For example, if updates occur only | |
172 | once per hour, then no explicit rate limiting is required, | |
173 | unless your system is already badly broken. The dcache | |
174 | subsystem takes this approach -- updates are guarded | |
175 | by a global lock, limiting their rate. | |
176 | ||
177 | c. Trusted update -- if updates can only be done manually by | |
178 | superuser or some other trusted user, then it might not | |
179 | be necessary to automatically limit them. The theory | |
180 | here is that superuser already has lots of ways to crash | |
181 | the machine. | |
182 | ||
183 | d. Use call_rcu_bh() rather than call_rcu(), in order to take | |
184 | advantage of call_rcu_bh()'s faster grace periods. | |
185 | ||
186 | e. Periodically invoke synchronize_rcu(), permitting a limited | |
187 | number of updates per grace period. | |
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188 | |
189 | 9. All RCU list-traversal primitives, which include | |
190 | list_for_each_rcu(), list_for_each_entry_rcu(), | |
191 | list_for_each_continue_rcu(), and list_for_each_safe_rcu(), | |
192 | must be within an RCU read-side critical section. RCU | |
193 | read-side critical sections are delimited by rcu_read_lock() | |
194 | and rcu_read_unlock(), or by similar primitives such as | |
195 | rcu_read_lock_bh() and rcu_read_unlock_bh(). | |
196 | ||
197 | Use of the _rcu() list-traversal primitives outside of an | |
198 | RCU read-side critical section causes no harm other than | |
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199 | a slight performance degradation on Alpha CPUs. It can |
200 | also be quite helpful in reducing code bloat when common | |
201 | code is shared between readers and updaters. | |
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202 | |
203 | 10. Conversely, if you are in an RCU read-side critical section, | |
204 | you -must- use the "_rcu()" variants of the list macros. | |
205 | Failing to do so will break Alpha and confuse people reading | |
206 | your code. | |
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207 | |
208 | 11. Note that synchronize_rcu() -only- guarantees to wait until | |
209 | all currently executing rcu_read_lock()-protected RCU read-side | |
210 | critical sections complete. It does -not- necessarily guarantee | |
211 | that all currently running interrupts, NMIs, preempt_disable() | |
212 | code, or idle loops will complete. Therefore, if you do not have | |
213 | rcu_read_lock()-protected read-side critical sections, do -not- | |
214 | use synchronize_rcu(). | |
215 | ||
216 | If you want to wait for some of these other things, you might | |
217 | instead need to use synchronize_irq() or synchronize_sched(). | |
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218 | |
219 | 12. Any lock acquired by an RCU callback must be acquired elsewhere | |
220 | with irq disabled, e.g., via spin_lock_irqsave(). Failing to | |
221 | disable irq on a given acquisition of that lock will result in | |
222 | deadlock as soon as the RCU callback happens to interrupt that | |
223 | acquisition's critical section. | |
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224 | |
225 | 13. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu()) | |
226 | may only be invoked from process context. Unlike other forms of | |
227 | RCU, it -is- permissible to block in an SRCU read-side critical | |
228 | section (demarked by srcu_read_lock() and srcu_read_unlock()), | |
229 | hence the "SRCU": "sleepable RCU". Please note that if you | |
230 | don't need to sleep in read-side critical sections, you should | |
231 | be using RCU rather than SRCU, because RCU is almost always | |
232 | faster and easier to use than is SRCU. | |
233 | ||
234 | Also unlike other forms of RCU, explicit initialization | |
235 | and cleanup is required via init_srcu_struct() and | |
236 | cleanup_srcu_struct(). These are passed a "struct srcu_struct" | |
237 | that defines the scope of a given SRCU domain. Once initialized, | |
238 | the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock() | |
239 | and synchronize_srcu(). A given synchronize_srcu() waits only | |
240 | for SRCU read-side critical sections governed by srcu_read_lock() | |
241 | and srcu_read_unlock() calls that have been passd the same | |
242 | srcu_struct. This property is what makes sleeping read-side | |
243 | critical sections tolerable -- a given subsystem delays only | |
244 | its own updates, not those of other subsystems using SRCU. | |
245 | Therefore, SRCU is less prone to OOM the system than RCU would | |
246 | be if RCU's read-side critical sections were permitted to | |
247 | sleep. | |
248 | ||
249 | The ability to sleep in read-side critical sections does not | |
250 | come for free. First, corresponding srcu_read_lock() and | |
251 | srcu_read_unlock() calls must be passed the same srcu_struct. | |
252 | Second, grace-period-detection overhead is amortized only | |
253 | over those updates sharing a given srcu_struct, rather than | |
254 | being globally amortized as they are for other forms of RCU. | |
255 | Therefore, SRCU should be used in preference to rw_semaphore | |
256 | only in extremely read-intensive situations, or in situations | |
257 | requiring SRCU's read-side deadlock immunity or low read-side | |
258 | realtime latency. | |
259 | ||
260 | Note that, rcu_assign_pointer() and rcu_dereference() relate to | |
261 | SRCU just as they do to other forms of RCU. |