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 bid 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. If you don't believe me, see: 133 134 http://www.openvms.compaq.com/wizard/wiz_2637.html 135 136 The rcu_dereference() primitive is also an excellent 137 documentation aid, letting the person reading the 138 code know exactly which pointers are protected by RCU. 139 Please note that compilers can also reorder code, and 140 they are becoming increasingly aggressive about doing 141 just that. The rcu_dereference() primitive therefore also 142 prevents destructive compiler optimizations. However, 143 with a bit of devious creativity, it is possible to 144 mishandle the return value from rcu_dereference(). 145 Please see rcu_dereference.txt in this directory for 146 more information. 147 148 The rcu_dereference() primitive is used by the 149 various "_rcu()" list-traversal primitives, such 150 as the list_for_each_entry_rcu(). Note that it is 151 perfectly legal (if redundant) for update-side code to 152 use rcu_dereference() and the "_rcu()" list-traversal 153 primitives. This is particularly useful in code that 154 is common to readers and updaters. However, lockdep 155 will complain if you access rcu_dereference() outside 156 of an RCU read-side critical section. See lockdep.txt 157 to learn what to do about this. 158 159 Of course, neither rcu_dereference() nor the "_rcu()" 160 list-traversal primitives can substitute for a good 161 concurrency design coordinating among multiple updaters. 162 163 b. If the list macros are being used, the list_add_tail_rcu() 164 and list_add_rcu() primitives must be used in order 165 to prevent weakly ordered machines from misordering 166 structure initialization and pointer planting. 167 Similarly, if the hlist macros are being used, the 168 hlist_add_head_rcu() primitive is required. 169 170 c. If the list macros are being used, the list_del_rcu() 171 primitive must be used to keep list_del()'s pointer 172 poisoning from inflicting toxic effects on concurrent 173 readers. Similarly, if the hlist macros are being used, 174 the hlist_del_rcu() primitive is required. 175 176 The list_replace_rcu() and hlist_replace_rcu() primitives 177 may be used to replace an old structure with a new one 178 in their respective types of RCU-protected lists. 179 180 d. Rules similar to (4b) and (4c) apply to the "hlist_nulls" 181 type of RCU-protected linked lists. 182 183 e. Updates must ensure that initialization of a given 184 structure happens before pointers to that structure are 185 publicized. Use the rcu_assign_pointer() primitive 186 when publicizing a pointer to a structure that can 187 be traversed by an RCU read-side critical section. 188 1895. If call_rcu() or call_srcu() is used, the callback function will 190 be called from softirq context. In particular, it cannot block. 191 1926. Since synchronize_rcu() can block, it cannot be called 193 from any sort of irq context. The same rule applies 194 for synchronize_srcu(), synchronize_rcu_expedited(), and 195 synchronize_srcu_expedited(). 196 197 The expedited forms of these primitives have the same semantics 198 as the non-expedited forms, but expediting is both expensive and 199 (with the exception of synchronize_srcu_expedited()) unfriendly 200 to real-time workloads. Use of the expedited primitives should 201 be restricted to rare configuration-change operations that would 202 not normally be undertaken while a real-time workload is running. 203 However, real-time workloads can use rcupdate.rcu_normal kernel 204 boot parameter to completely disable expedited grace periods, 205 though this might have performance implications. 206 207 In particular, if you find yourself invoking one of the expedited 208 primitives repeatedly in a loop, please do everyone a favor: 209 Restructure your code so that it batches the updates, allowing 210 a single non-expedited primitive to cover the entire batch. 211 This will very likely be faster than the loop containing the 212 expedited primitive, and will be much much easier on the rest 213 of the system, especially to real-time workloads running on 214 the rest of the system. 215 2167. As of v4.20, a given kernel implements only one RCU flavor, 217 which is RCU-sched for PREEMPT=n and RCU-preempt for PREEMPT=y. 218 If the updater uses call_rcu() or synchronize_rcu(), 219 then the corresponding readers my use rcu_read_lock() and 220 rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(), 221 or any pair of primitives that disables and re-enables preemption, 222 for example, rcu_read_lock_sched() and rcu_read_unlock_sched(). 223 If the updater uses synchronize_srcu() or call_srcu(), 224 then the corresponding readers must use srcu_read_lock() and 225 srcu_read_unlock(), and with the same srcu_struct. The rules for 226 the expedited primitives are the same as for their non-expedited 227 counterparts. Mixing things up will result in confusion and 228 broken kernels, and has even resulted in an exploitable security 229 issue. 230 231 One exception to this rule: rcu_read_lock() and rcu_read_unlock() 232 may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh() 233 in cases where local bottom halves are already known to be 234 disabled, for example, in irq or softirq context. Commenting 235 such cases is a must, of course! And the jury is still out on 236 whether the increased speed is worth it. 237 2388. Although synchronize_rcu() is slower than is call_rcu(), it 239 usually results in simpler code. So, unless update performance is 240 critically important, the updaters cannot block, or the latency of 241 synchronize_rcu() is visible from userspace, synchronize_rcu() 242 should be used in preference to call_rcu(). Furthermore, 243 kfree_rcu() usually results in even simpler code than does 244 synchronize_rcu() without synchronize_rcu()'s multi-millisecond 245 latency. So please take advantage of kfree_rcu()'s "fire and 246 forget" memory-freeing capabilities where it applies. 247 248 An especially important property of the synchronize_rcu() 249 primitive is that it automatically self-limits: if grace periods 250 are delayed for whatever reason, then the synchronize_rcu() 251 primitive will correspondingly delay updates. In contrast, 252 code using call_rcu() should explicitly limit update rate in 253 cases where grace periods are delayed, as failing to do so can 254 result in excessive realtime latencies or even OOM conditions. 255 256 Ways of gaining this self-limiting property when using call_rcu() 257 include: 258 259 a. Keeping a count of the number of data-structure elements 260 used by the RCU-protected data structure, including 261 those waiting for a grace period to elapse. Enforce a 262 limit on this number, stalling updates as needed to allow 263 previously deferred frees to complete. Alternatively, 264 limit only the number awaiting deferred free rather than 265 the total number of elements. 266 267 One way to stall the updates is to acquire the update-side 268 mutex. (Don't try this with a spinlock -- other CPUs 269 spinning on the lock could prevent the grace period 270 from ever ending.) Another way to stall the updates 271 is for the updates to use a wrapper function around 272 the memory allocator, so that this wrapper function 273 simulates OOM when there is too much memory awaiting an 274 RCU grace period. There are of course many other 275 variations on this theme. 276 277 b. Limiting update rate. For example, if updates occur only 278 once per hour, then no explicit rate limiting is 279 required, unless your system is already badly broken. 280 Older versions of the dcache subsystem take this approach, 281 guarding updates with a global lock, limiting their rate. 282 283 c. Trusted update -- if updates can only be done manually by 284 superuser or some other trusted user, then it might not 285 be necessary to automatically limit them. The theory 286 here is that superuser already has lots of ways to crash 287 the machine. 288 289 d. Periodically invoke synchronize_rcu(), permitting a limited 290 number of updates per grace period. 291 292 The same cautions apply to call_srcu() and kfree_rcu(). 293 294 Note that although these primitives do take action to avoid memory 295 exhaustion when any given CPU has too many callbacks, a determined 296 user could still exhaust memory. This is especially the case 297 if a system with a large number of CPUs has been configured to 298 offload all of its RCU callbacks onto a single CPU, or if the 299 system has relatively little free memory. 300 3019. All RCU list-traversal primitives, which include 302 rcu_dereference(), list_for_each_entry_rcu(), and 303 list_for_each_safe_rcu(), must be either within an RCU read-side 304 critical section or must be protected by appropriate update-side 305 locks. RCU read-side critical sections are delimited by 306 rcu_read_lock() and rcu_read_unlock(), or by similar primitives 307 such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which 308 case the matching rcu_dereference() primitive must be used in 309 order to keep lockdep happy, in this case, rcu_dereference_bh(). 310 311 The reason that it is permissible to use RCU list-traversal 312 primitives when the update-side lock is held is that doing so 313 can be quite helpful in reducing code bloat when common code is 314 shared between readers and updaters. Additional primitives 315 are provided for this case, as discussed in lockdep.txt. 316 317 One exception to this rule is when data is only ever added to 318 the linked data structure, and is never removed during any 319 time that readers might be accessing that structure. In such 320 cases, READ_ONCE() may be used in place of rcu_dereference() 321 and the read-side markers (rcu_read_lock() and rcu_read_unlock(), 322 for example) may be omitted. 323 32410. Conversely, if you are in an RCU read-side critical section, 325 and you don't hold the appropriate update-side lock, you -must- 326 use the "_rcu()" variants of the list macros. Failing to do so 327 will break Alpha, cause aggressive compilers to generate bad code, 328 and confuse people trying to read your code. 329 33011. Any lock acquired by an RCU callback must be acquired elsewhere 331 with softirq disabled, e.g., via spin_lock_irqsave(), 332 spin_lock_bh(), etc. Failing to disable softirq on a given 333 acquisition of that lock will result in deadlock as soon as 334 the RCU softirq handler happens to run your RCU callback while 335 interrupting that acquisition's critical section. 336 33712. RCU callbacks can be and are executed in parallel. In many cases, 338 the callback code simply wrappers around kfree(), so that this 339 is not an issue (or, more accurately, to the extent that it is 340 an issue, the memory-allocator locking handles it). However, 341 if the callbacks do manipulate a shared data structure, they 342 must use whatever locking or other synchronization is required 343 to safely access and/or modify that data structure. 344 345 Do not assume that RCU callbacks will be executed on the same 346 CPU that executed the corresponding call_rcu() or call_srcu(). 347 For example, if a given CPU goes offline while having an RCU 348 callback pending, then that RCU callback will execute on some 349 surviving CPU. (If this was not the case, a self-spawning RCU 350 callback would prevent the victim CPU from ever going offline.) 351 Furthermore, CPUs designated by rcu_nocbs= might well -always- 352 have their RCU callbacks executed on some other CPUs, in fact, 353 for some real-time workloads, this is the whole point of using 354 the rcu_nocbs= kernel boot parameter. 355 35613. Unlike other forms of RCU, it -is- permissible to block in an 357 SRCU read-side critical section (demarked by srcu_read_lock() 358 and srcu_read_unlock()), hence the "SRCU": "sleepable RCU". 359 Please note that if you don't need to sleep in read-side critical 360 sections, you should be using RCU rather than SRCU, because RCU 361 is almost always faster and easier to use than is SRCU. 362 363 Also unlike other forms of RCU, explicit initialization and 364 cleanup is required either at build time via DEFINE_SRCU() 365 or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct() 366 and cleanup_srcu_struct(). These last two are passed a 367 "struct srcu_struct" that defines the scope of a given 368 SRCU domain. Once initialized, the srcu_struct is passed 369 to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(), 370 synchronize_srcu_expedited(), and call_srcu(). A given 371 synchronize_srcu() waits only for SRCU read-side critical 372 sections governed by srcu_read_lock() and srcu_read_unlock() 373 calls that have been passed the same srcu_struct. This property 374 is what makes sleeping read-side critical sections tolerable -- 375 a given subsystem delays only its own updates, not those of other 376 subsystems using SRCU. Therefore, SRCU is less prone to OOM the 377 system than RCU would be if RCU's read-side critical sections 378 were permitted to sleep. 379 380 The ability to sleep in read-side critical sections does not 381 come for free. First, corresponding srcu_read_lock() and 382 srcu_read_unlock() calls must be passed the same srcu_struct. 383 Second, grace-period-detection overhead is amortized only 384 over those updates sharing a given srcu_struct, rather than 385 being globally amortized as they are for other forms of RCU. 386 Therefore, SRCU should be used in preference to rw_semaphore 387 only in extremely read-intensive situations, or in situations 388 requiring SRCU's read-side deadlock immunity or low read-side 389 realtime latency. You should also consider percpu_rw_semaphore 390 when you need lightweight readers. 391 392 SRCU's expedited primitive (synchronize_srcu_expedited()) 393 never sends IPIs to other CPUs, so it is easier on 394 real-time workloads than is synchronize_rcu_expedited(). 395 396 Note that rcu_assign_pointer() relates to SRCU just as it does to 397 other forms of RCU, but instead of rcu_dereference() you should 398 use srcu_dereference() in order to avoid lockdep splats. 399 40014. The whole point of call_rcu(), synchronize_rcu(), and friends 401 is to wait until all pre-existing readers have finished before 402 carrying out some otherwise-destructive operation. It is 403 therefore critically important to -first- remove any path 404 that readers can follow that could be affected by the 405 destructive operation, and -only- -then- invoke call_rcu(), 406 synchronize_rcu(), or friends. 407 408 Because these primitives only wait for pre-existing readers, it 409 is the caller's responsibility to guarantee that any subsequent 410 readers will execute safely. 411 41215. The various RCU read-side primitives do -not- necessarily contain 413 memory barriers. You should therefore plan for the CPU 414 and the compiler to freely reorder code into and out of RCU 415 read-side critical sections. It is the responsibility of the 416 RCU update-side primitives to deal with this. 417 418 For SRCU readers, you can use smp_mb__after_srcu_read_unlock() 419 immediately after an srcu_read_unlock() to get a full barrier. 420 42116. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the 422 __rcu sparse checks to validate your RCU code. These can help 423 find problems as follows: 424 425 CONFIG_PROVE_LOCKING: 426 check that accesses to RCU-protected data 427 structures are carried out under the proper RCU 428 read-side critical section, while holding the right 429 combination of locks, or whatever other conditions 430 are appropriate. 431 432 CONFIG_DEBUG_OBJECTS_RCU_HEAD: 433 check that you don't pass the 434 same object to call_rcu() (or friends) before an RCU 435 grace period has elapsed since the last time that you 436 passed that same object to call_rcu() (or friends). 437 438 __rcu sparse checks: 439 tag the pointer to the RCU-protected data 440 structure with __rcu, and sparse will warn you if you 441 access that pointer without the services of one of the 442 variants of rcu_dereference(). 443 444 These debugging aids can help you find problems that are 445 otherwise extremely difficult to spot. 446 44717. If you register a callback using call_rcu() or call_srcu(), and 448 pass in a function defined within a loadable module, then it in 449 necessary to wait for all pending callbacks to be invoked after 450 the last invocation and before unloading that module. Note that 451 it is absolutely -not- sufficient to wait for a grace period! 452 The current (say) synchronize_rcu() implementation is -not- 453 guaranteed to wait for callbacks registered on other CPUs. 454 Or even on the current CPU if that CPU recently went offline 455 and came back online. 456 457 You instead need to use one of the barrier functions: 458 459 - call_rcu() -> rcu_barrier() 460 - call_srcu() -> srcu_barrier() 461 462 However, these barrier functions are absolutely -not- guaranteed 463 to wait for a grace period. In fact, if there are no call_rcu() 464 callbacks waiting anywhere in the system, rcu_barrier() is within 465 its rights to return immediately. 466 467 So if you need to wait for both an RCU grace period and for 468 all pre-existing call_rcu() callbacks, you will need to execute 469 both rcu_barrier() and synchronize_rcu(), if necessary, using 470 something like workqueues to to execute them concurrently. 471 472 See rcubarrier.txt for more information. 473