1.. SPDX-License-Identifier: GPL-2.0 2 3================================ 4Review Checklist for RCU Patches 5================================ 6 7 8This document contains a checklist for producing and reviewing patches 9that make use of RCU. Violating any of the rules listed below will 10result in the same sorts of problems that leaving out a locking primitive 11would cause. This list is based on experiences reviewing such patches 12over a rather long period of time, but improvements are always welcome! 13 140. Is RCU being applied to a read-mostly situation? If the data 15 structure is updated more than about 10% of the time, then you 16 should strongly consider some other approach, unless detailed 17 performance measurements show that RCU is nonetheless the right 18 tool for the job. Yes, RCU does reduce read-side overhead by 19 increasing write-side overhead, which is exactly why normal uses 20 of RCU will do much more reading than updating. 21 22 Another exception is where performance is not an issue, and RCU 23 provides a simpler implementation. An example of this situation 24 is the dynamic NMI code in the Linux 2.6 kernel, at least on 25 architectures where NMIs are rare. 26 27 Yet another exception is where the low real-time latency of RCU's 28 read-side primitives is critically important. 29 30 One final exception is where RCU readers are used to prevent 31 the ABA problem (https://en.wikipedia.org/wiki/ABA_problem) 32 for lockless updates. This does result in the mildly 33 counter-intuitive situation where rcu_read_lock() and 34 rcu_read_unlock() are used to protect updates, however, this 35 approach provides the same potential simplifications that garbage 36 collectors do. 37 381. Does the update code have proper mutual exclusion? 39 40 RCU does allow -readers- to run (almost) naked, but -writers- must 41 still use some sort of mutual exclusion, such as: 42 43 a. locking, 44 b. atomic operations, or 45 c. restricting updates to a single task. 46 47 If you choose #b, be prepared to describe how you have handled 48 memory barriers on weakly ordered machines (pretty much all of 49 them -- even x86 allows later loads to be reordered to precede 50 earlier stores), and be prepared to explain why this added 51 complexity is worthwhile. If you choose #c, be prepared to 52 explain how this single task does not become a major bottleneck on 53 big multiprocessor machines (for example, if the task is updating 54 information relating to itself that other tasks can read, there 55 by definition can be no bottleneck). Note that the definition 56 of "large" has changed significantly: Eight CPUs was "large" 57 in the year 2000, but a hundred CPUs was unremarkable in 2017. 58 592. Do the RCU read-side critical sections make proper use of 60 rcu_read_lock() and friends? These primitives are needed 61 to prevent grace periods from ending prematurely, which 62 could result in data being unceremoniously freed out from 63 under your read-side code, which can greatly increase the 64 actuarial risk of your kernel. 65 66 As a rough rule of thumb, any dereference of an RCU-protected 67 pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(), 68 rcu_read_lock_sched(), or by the appropriate update-side lock. 69 Disabling of preemption can serve as rcu_read_lock_sched(), but 70 is less readable and prevents lockdep from detecting locking issues. 71 72 Letting RCU-protected pointers "leak" out of an RCU read-side 73 critical section is every bit as bad as letting them leak out 74 from under a lock. Unless, of course, you have arranged some 75 other means of protection, such as a lock or a reference count 76 -before- letting them out of the RCU read-side critical section. 77 783. Does the update code tolerate concurrent accesses? 79 80 The whole point of RCU is to permit readers to run without 81 any locks or atomic operations. This means that readers will 82 be running while updates are in progress. There are a number 83 of ways to handle this concurrency, depending on the situation: 84 85 a. Use the RCU variants of the list and hlist update 86 primitives to add, remove, and replace elements on 87 an RCU-protected list. Alternatively, use the other 88 RCU-protected data structures that have been added to 89 the Linux kernel. 90 91 This is almost always the best approach. 92 93 b. Proceed as in (a) above, but also maintain per-element 94 locks (that are acquired by both readers and writers) 95 that guard per-element state. Of course, fields that 96 the readers refrain from accessing can be guarded by 97 some other lock acquired only by updaters, if desired. 98 99 This works quite well, also. 100 101 c. Make updates appear atomic to readers. For example, 102 pointer updates to properly aligned fields will 103 appear atomic, as will individual atomic primitives. 104 Sequences of operations performed under a lock will -not- 105 appear to be atomic to RCU readers, nor will sequences 106 of multiple atomic primitives. 107 108 This can work, but is starting to get a bit tricky. 109 110 d. Carefully order the updates and the reads so that 111 readers see valid data at all phases of the update. 112 This is often more difficult than it sounds, especially 113 given modern CPUs' tendency to reorder memory references. 114 One must usually liberally sprinkle memory barriers 115 (smp_wmb(), smp_rmb(), smp_mb()) through the code, 116 making it difficult to understand and to test. 117 118 It is usually better to group the changing data into 119 a separate structure, so that the change may be made 120 to appear atomic by updating a pointer to reference 121 a new structure containing updated values. 122 1234. Weakly ordered CPUs pose special challenges. Almost all CPUs 124 are weakly ordered -- even x86 CPUs allow later loads to be 125 reordered to precede earlier stores. RCU code must take all of 126 the following measures to prevent memory-corruption problems: 127 128 a. Readers must maintain proper ordering of their memory 129 accesses. The rcu_dereference() primitive ensures that 130 the CPU picks up the pointer before it picks up the data 131 that the pointer points to. This really is necessary 132 on Alpha CPUs. 133 134 The rcu_dereference() primitive is also an excellent 135 documentation aid, letting the person reading the 136 code know exactly which pointers are protected by RCU. 137 Please note that compilers can also reorder code, and 138 they are becoming increasingly aggressive about doing 139 just that. The rcu_dereference() primitive therefore also 140 prevents destructive compiler optimizations. However, 141 with a bit of devious creativity, it is possible to 142 mishandle the return value from rcu_dereference(). 143 Please see rcu_dereference.txt in this directory for 144 more information. 145 146 The rcu_dereference() primitive is used by the 147 various "_rcu()" list-traversal primitives, such 148 as the list_for_each_entry_rcu(). Note that it is 149 perfectly legal (if redundant) for update-side code to 150 use rcu_dereference() and the "_rcu()" list-traversal 151 primitives. This is particularly useful in code that 152 is common to readers and updaters. However, lockdep 153 will complain if you access rcu_dereference() outside 154 of an RCU read-side critical section. See lockdep.txt 155 to learn what to do about this. 156 157 Of course, neither rcu_dereference() nor the "_rcu()" 158 list-traversal primitives can substitute for a good 159 concurrency design coordinating among multiple updaters. 160 161 b. If the list macros are being used, the list_add_tail_rcu() 162 and list_add_rcu() primitives must be used in order 163 to prevent weakly ordered machines from misordering 164 structure initialization and pointer planting. 165 Similarly, if the hlist macros are being used, the 166 hlist_add_head_rcu() primitive is required. 167 168 c. If the list macros are being used, the list_del_rcu() 169 primitive must be used to keep list_del()'s pointer 170 poisoning from inflicting toxic effects on concurrent 171 readers. Similarly, if the hlist macros are being used, 172 the hlist_del_rcu() primitive is required. 173 174 The list_replace_rcu() and hlist_replace_rcu() primitives 175 may be used to replace an old structure with a new one 176 in their respective types of RCU-protected lists. 177 178 d. Rules similar to (4b) and (4c) apply to the "hlist_nulls" 179 type of RCU-protected linked lists. 180 181 e. Updates must ensure that initialization of a given 182 structure happens before pointers to that structure are 183 publicized. Use the rcu_assign_pointer() primitive 184 when publicizing a pointer to a structure that can 185 be traversed by an RCU read-side critical section. 186 1875. If call_rcu() or call_srcu() is used, the callback function will 188 be called from softirq context. In particular, it cannot block. 189 1906. Since synchronize_rcu() can block, it cannot be called 191 from any sort of irq context. The same rule applies 192 for synchronize_srcu(), synchronize_rcu_expedited(), and 193 synchronize_srcu_expedited(). 194 195 The expedited forms of these primitives have the same semantics 196 as the non-expedited forms, but expediting is both expensive and 197 (with the exception of synchronize_srcu_expedited()) unfriendly 198 to real-time workloads. Use of the expedited primitives should 199 be restricted to rare configuration-change operations that would 200 not normally be undertaken while a real-time workload is running. 201 However, real-time workloads can use rcupdate.rcu_normal kernel 202 boot parameter to completely disable expedited grace periods, 203 though this might have performance implications. 204 205 In particular, if you find yourself invoking one of the expedited 206 primitives repeatedly in a loop, please do everyone a favor: 207 Restructure your code so that it batches the updates, allowing 208 a single non-expedited primitive to cover the entire batch. 209 This will very likely be faster than the loop containing the 210 expedited primitive, and will be much much easier on the rest 211 of the system, especially to real-time workloads running on 212 the rest of the system. 213 2147. As of v4.20, a given kernel implements only one RCU flavor, 215 which is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y. 216 If the updater uses call_rcu() or synchronize_rcu(), 217 then the corresponding readers may use rcu_read_lock() and 218 rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(), 219 or any pair of primitives that disables and re-enables preemption, 220 for example, rcu_read_lock_sched() and rcu_read_unlock_sched(). 221 If the updater uses synchronize_srcu() or call_srcu(), 222 then the corresponding readers must use srcu_read_lock() and 223 srcu_read_unlock(), and with the same srcu_struct. The rules for 224 the expedited primitives are the same as for their non-expedited 225 counterparts. Mixing things up will result in confusion and 226 broken kernels, and has even resulted in an exploitable security 227 issue. 228 229 One exception to this rule: rcu_read_lock() and rcu_read_unlock() 230 may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh() 231 in cases where local bottom halves are already known to be 232 disabled, for example, in irq or softirq context. Commenting 233 such cases is a must, of course! And the jury is still out on 234 whether the increased speed is worth it. 235 2368. Although synchronize_rcu() is slower than is call_rcu(), it 237 usually results in simpler code. So, unless update performance is 238 critically important, the updaters cannot block, or the latency of 239 synchronize_rcu() is visible from userspace, synchronize_rcu() 240 should be used in preference to call_rcu(). Furthermore, 241 kfree_rcu() usually results in even simpler code than does 242 synchronize_rcu() without synchronize_rcu()'s multi-millisecond 243 latency. So please take advantage of kfree_rcu()'s "fire and 244 forget" memory-freeing capabilities where it applies. 245 246 An especially important property of the synchronize_rcu() 247 primitive is that it automatically self-limits: if grace periods 248 are delayed for whatever reason, then the synchronize_rcu() 249 primitive will correspondingly delay updates. In contrast, 250 code using call_rcu() should explicitly limit update rate in 251 cases where grace periods are delayed, as failing to do so can 252 result in excessive realtime latencies or even OOM conditions. 253 254 Ways of gaining this self-limiting property when using call_rcu() 255 include: 256 257 a. Keeping a count of the number of data-structure elements 258 used by the RCU-protected data structure, including 259 those waiting for a grace period to elapse. Enforce a 260 limit on this number, stalling updates as needed to allow 261 previously deferred frees to complete. Alternatively, 262 limit only the number awaiting deferred free rather than 263 the total number of elements. 264 265 One way to stall the updates is to acquire the update-side 266 mutex. (Don't try this with a spinlock -- other CPUs 267 spinning on the lock could prevent the grace period 268 from ever ending.) Another way to stall the updates 269 is for the updates to use a wrapper function around 270 the memory allocator, so that this wrapper function 271 simulates OOM when there is too much memory awaiting an 272 RCU grace period. There are of course many other 273 variations on this theme. 274 275 b. Limiting update rate. For example, if updates occur only 276 once per hour, then no explicit rate limiting is 277 required, unless your system is already badly broken. 278 Older versions of the dcache subsystem take this approach, 279 guarding updates with a global lock, limiting their rate. 280 281 c. Trusted update -- if updates can only be done manually by 282 superuser or some other trusted user, then it might not 283 be necessary to automatically limit them. The theory 284 here is that superuser already has lots of ways to crash 285 the machine. 286 287 d. Periodically invoke synchronize_rcu(), permitting a limited 288 number of updates per grace period. 289 290 The same cautions apply to call_srcu() and kfree_rcu(). 291 292 Note that although these primitives do take action to avoid memory 293 exhaustion when any given CPU has too many callbacks, a determined 294 user could still exhaust memory. This is especially the case 295 if a system with a large number of CPUs has been configured to 296 offload all of its RCU callbacks onto a single CPU, or if the 297 system has relatively little free memory. 298 2999. All RCU list-traversal primitives, which include 300 rcu_dereference(), list_for_each_entry_rcu(), and 301 list_for_each_safe_rcu(), must be either within an RCU read-side 302 critical section or must be protected by appropriate update-side 303 locks. RCU read-side critical sections are delimited by 304 rcu_read_lock() and rcu_read_unlock(), or by similar primitives 305 such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which 306 case the matching rcu_dereference() primitive must be used in 307 order to keep lockdep happy, in this case, rcu_dereference_bh(). 308 309 The reason that it is permissible to use RCU list-traversal 310 primitives when the update-side lock is held is that doing so 311 can be quite helpful in reducing code bloat when common code is 312 shared between readers and updaters. Additional primitives 313 are provided for this case, as discussed in lockdep.txt. 314 315 One exception to this rule is when data is only ever added to 316 the linked data structure, and is never removed during any 317 time that readers might be accessing that structure. In such 318 cases, READ_ONCE() may be used in place of rcu_dereference() 319 and the read-side markers (rcu_read_lock() and rcu_read_unlock(), 320 for example) may be omitted. 321 32210. Conversely, if you are in an RCU read-side critical section, 323 and you don't hold the appropriate update-side lock, you -must- 324 use the "_rcu()" variants of the list macros. Failing to do so 325 will break Alpha, cause aggressive compilers to generate bad code, 326 and confuse people trying to read your code. 327 32811. Any lock acquired by an RCU callback must be acquired elsewhere 329 with softirq disabled, e.g., via spin_lock_irqsave(), 330 spin_lock_bh(), etc. Failing to disable softirq on a given 331 acquisition of that lock will result in deadlock as soon as 332 the RCU softirq handler happens to run your RCU callback while 333 interrupting that acquisition's critical section. 334 33512. RCU callbacks can be and are executed in parallel. In many cases, 336 the callback code simply wrappers around kfree(), so that this 337 is not an issue (or, more accurately, to the extent that it is 338 an issue, the memory-allocator locking handles it). However, 339 if the callbacks do manipulate a shared data structure, they 340 must use whatever locking or other synchronization is required 341 to safely access and/or modify that data structure. 342 343 Do not assume that RCU callbacks will be executed on the same 344 CPU that executed the corresponding call_rcu() or call_srcu(). 345 For example, if a given CPU goes offline while having an RCU 346 callback pending, then that RCU callback will execute on some 347 surviving CPU. (If this was not the case, a self-spawning RCU 348 callback would prevent the victim CPU from ever going offline.) 349 Furthermore, CPUs designated by rcu_nocbs= might well -always- 350 have their RCU callbacks executed on some other CPUs, in fact, 351 for some real-time workloads, this is the whole point of using 352 the rcu_nocbs= kernel boot parameter. 353 35413. Unlike other forms of RCU, it -is- permissible to block in an 355 SRCU read-side critical section (demarked by srcu_read_lock() 356 and srcu_read_unlock()), hence the "SRCU": "sleepable RCU". 357 Please note that if you don't need to sleep in read-side critical 358 sections, you should be using RCU rather than SRCU, because RCU 359 is almost always faster and easier to use than is SRCU. 360 361 Also unlike other forms of RCU, explicit initialization and 362 cleanup is required either at build time via DEFINE_SRCU() 363 or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct() 364 and cleanup_srcu_struct(). These last two are passed a 365 "struct srcu_struct" that defines the scope of a given 366 SRCU domain. Once initialized, the srcu_struct is passed 367 to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(), 368 synchronize_srcu_expedited(), and call_srcu(). A given 369 synchronize_srcu() waits only for SRCU read-side critical 370 sections governed by srcu_read_lock() and srcu_read_unlock() 371 calls that have been passed the same srcu_struct. This property 372 is what makes sleeping read-side critical sections tolerable -- 373 a given subsystem delays only its own updates, not those of other 374 subsystems using SRCU. Therefore, SRCU is less prone to OOM the 375 system than RCU would be if RCU's read-side critical sections 376 were permitted to sleep. 377 378 The ability to sleep in read-side critical sections does not 379 come for free. First, corresponding srcu_read_lock() and 380 srcu_read_unlock() calls must be passed the same srcu_struct. 381 Second, grace-period-detection overhead is amortized only 382 over those updates sharing a given srcu_struct, rather than 383 being globally amortized as they are for other forms of RCU. 384 Therefore, SRCU should be used in preference to rw_semaphore 385 only in extremely read-intensive situations, or in situations 386 requiring SRCU's read-side deadlock immunity or low read-side 387 realtime latency. You should also consider percpu_rw_semaphore 388 when you need lightweight readers. 389 390 SRCU's expedited primitive (synchronize_srcu_expedited()) 391 never sends IPIs to other CPUs, so it is easier on 392 real-time workloads than is synchronize_rcu_expedited(). 393 394 Note that rcu_assign_pointer() relates to SRCU just as it does to 395 other forms of RCU, but instead of rcu_dereference() you should 396 use srcu_dereference() in order to avoid lockdep splats. 397 39814. The whole point of call_rcu(), synchronize_rcu(), and friends 399 is to wait until all pre-existing readers have finished before 400 carrying out some otherwise-destructive operation. It is 401 therefore critically important to -first- remove any path 402 that readers can follow that could be affected by the 403 destructive operation, and -only- -then- invoke call_rcu(), 404 synchronize_rcu(), or friends. 405 406 Because these primitives only wait for pre-existing readers, it 407 is the caller's responsibility to guarantee that any subsequent 408 readers will execute safely. 409 41015. The various RCU read-side primitives do -not- necessarily contain 411 memory barriers. You should therefore plan for the CPU 412 and the compiler to freely reorder code into and out of RCU 413 read-side critical sections. It is the responsibility of the 414 RCU update-side primitives to deal with this. 415 416 For SRCU readers, you can use smp_mb__after_srcu_read_unlock() 417 immediately after an srcu_read_unlock() to get a full barrier. 418 41916. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the 420 __rcu sparse checks to validate your RCU code. These can help 421 find problems as follows: 422 423 CONFIG_PROVE_LOCKING: 424 check that accesses to RCU-protected data 425 structures are carried out under the proper RCU 426 read-side critical section, while holding the right 427 combination of locks, or whatever other conditions 428 are appropriate. 429 430 CONFIG_DEBUG_OBJECTS_RCU_HEAD: 431 check that you don't pass the 432 same object to call_rcu() (or friends) before an RCU 433 grace period has elapsed since the last time that you 434 passed that same object to call_rcu() (or friends). 435 436 __rcu sparse checks: 437 tag the pointer to the RCU-protected data 438 structure with __rcu, and sparse will warn you if you 439 access that pointer without the services of one of the 440 variants of rcu_dereference(). 441 442 These debugging aids can help you find problems that are 443 otherwise extremely difficult to spot. 444 44517. If you register a callback using call_rcu() or call_srcu(), and 446 pass in a function defined within a loadable module, then it in 447 necessary to wait for all pending callbacks to be invoked after 448 the last invocation and before unloading that module. Note that 449 it is absolutely -not- sufficient to wait for a grace period! 450 The current (say) synchronize_rcu() implementation is -not- 451 guaranteed to wait for callbacks registered on other CPUs. 452 Or even on the current CPU if that CPU recently went offline 453 and came back online. 454 455 You instead need to use one of the barrier functions: 456 457 - call_rcu() -> rcu_barrier() 458 - call_srcu() -> srcu_barrier() 459 460 However, these barrier functions are absolutely -not- guaranteed 461 to wait for a grace period. In fact, if there are no call_rcu() 462 callbacks waiting anywhere in the system, rcu_barrier() is within 463 its rights to return immediately. 464 465 So if you need to wait for both an RCU grace period and for 466 all pre-existing call_rcu() callbacks, you will need to execute 467 both rcu_barrier() and synchronize_rcu(), if necessary, using 468 something like workqueues to to execute them concurrently. 469 470 See rcubarrier.txt for more information. 471