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, which 215 is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y. 216 If the updater uses call_rcu() or synchronize_rcu(), then 217 the corresponding readers may use: (1) rcu_read_lock() and 218 rcu_read_unlock(), (2) any pair of primitives that disables 219 and re-enables softirq, for example, rcu_read_lock_bh() and 220 rcu_read_unlock_bh(), or (3) any pair of primitives that disables 221 and re-enables preemption, for example, rcu_read_lock_sched() and 222 rcu_read_unlock_sched(). If the updater uses synchronize_srcu() 223 or call_srcu(), then the corresponding readers must use 224 srcu_read_lock() and srcu_read_unlock(), and with the same 225 srcu_struct. The rules for the expedited RCU grace-period-wait 226 primitives are the same as for their non-expedited counterparts. 227 228 If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(), 229 then the readers must refrain from executing voluntary 230 context switches, that is, from blocking. If the updater uses 231 call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then 232 the corresponding readers must use rcu_read_lock_trace() and 233 rcu_read_unlock_trace(). If an updater uses call_rcu_tasks_rude() 234 or synchronize_rcu_tasks_rude(), then the corresponding readers 235 must use anything that disables interrupts. 236 237 Mixing things up will result in confusion and broken kernels, and 238 has even resulted in an exploitable security issue. Therefore, 239 when using non-obvious pairs of primitives, commenting is 240 of course a must. One example of non-obvious pairing is 241 the XDP feature in networking, which calls BPF programs from 242 network-driver NAPI (softirq) context. BPF relies heavily on RCU 243 protection for its data structures, but because the BPF program 244 invocation happens entirely within a single local_bh_disable() 245 section in a NAPI poll cycle, this usage is safe. The reason 246 that this usage is safe is that readers can use anything that 247 disables BH when updaters use call_rcu() or synchronize_rcu(). 248 2498. Although synchronize_rcu() is slower than is call_rcu(), it 250 usually results in simpler code. So, unless update performance is 251 critically important, the updaters cannot block, or the latency of 252 synchronize_rcu() is visible from userspace, synchronize_rcu() 253 should be used in preference to call_rcu(). Furthermore, 254 kfree_rcu() usually results in even simpler code than does 255 synchronize_rcu() without synchronize_rcu()'s multi-millisecond 256 latency. So please take advantage of kfree_rcu()'s "fire and 257 forget" memory-freeing capabilities where it applies. 258 259 An especially important property of the synchronize_rcu() 260 primitive is that it automatically self-limits: if grace periods 261 are delayed for whatever reason, then the synchronize_rcu() 262 primitive will correspondingly delay updates. In contrast, 263 code using call_rcu() should explicitly limit update rate in 264 cases where grace periods are delayed, as failing to do so can 265 result in excessive realtime latencies or even OOM conditions. 266 267 Ways of gaining this self-limiting property when using call_rcu() 268 include: 269 270 a. Keeping a count of the number of data-structure elements 271 used by the RCU-protected data structure, including 272 those waiting for a grace period to elapse. Enforce a 273 limit on this number, stalling updates as needed to allow 274 previously deferred frees to complete. Alternatively, 275 limit only the number awaiting deferred free rather than 276 the total number of elements. 277 278 One way to stall the updates is to acquire the update-side 279 mutex. (Don't try this with a spinlock -- other CPUs 280 spinning on the lock could prevent the grace period 281 from ever ending.) Another way to stall the updates 282 is for the updates to use a wrapper function around 283 the memory allocator, so that this wrapper function 284 simulates OOM when there is too much memory awaiting an 285 RCU grace period. There are of course many other 286 variations on this theme. 287 288 b. Limiting update rate. For example, if updates occur only 289 once per hour, then no explicit rate limiting is 290 required, unless your system is already badly broken. 291 Older versions of the dcache subsystem take this approach, 292 guarding updates with a global lock, limiting their rate. 293 294 c. Trusted update -- if updates can only be done manually by 295 superuser or some other trusted user, then it might not 296 be necessary to automatically limit them. The theory 297 here is that superuser already has lots of ways to crash 298 the machine. 299 300 d. Periodically invoke synchronize_rcu(), permitting a limited 301 number of updates per grace period. 302 303 The same cautions apply to call_srcu() and kfree_rcu(). 304 305 Note that although these primitives do take action to avoid memory 306 exhaustion when any given CPU has too many callbacks, a determined 307 user could still exhaust memory. This is especially the case 308 if a system with a large number of CPUs has been configured to 309 offload all of its RCU callbacks onto a single CPU, or if the 310 system has relatively little free memory. 311 3129. All RCU list-traversal primitives, which include 313 rcu_dereference(), list_for_each_entry_rcu(), and 314 list_for_each_safe_rcu(), must be either within an RCU read-side 315 critical section or must be protected by appropriate update-side 316 locks. RCU read-side critical sections are delimited by 317 rcu_read_lock() and rcu_read_unlock(), or by similar primitives 318 such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which 319 case the matching rcu_dereference() primitive must be used in 320 order to keep lockdep happy, in this case, rcu_dereference_bh(). 321 322 The reason that it is permissible to use RCU list-traversal 323 primitives when the update-side lock is held is that doing so 324 can be quite helpful in reducing code bloat when common code is 325 shared between readers and updaters. Additional primitives 326 are provided for this case, as discussed in lockdep.txt. 327 328 One exception to this rule is when data is only ever added to 329 the linked data structure, and is never removed during any 330 time that readers might be accessing that structure. In such 331 cases, READ_ONCE() may be used in place of rcu_dereference() 332 and the read-side markers (rcu_read_lock() and rcu_read_unlock(), 333 for example) may be omitted. 334 33510. Conversely, if you are in an RCU read-side critical section, 336 and you don't hold the appropriate update-side lock, you -must- 337 use the "_rcu()" variants of the list macros. Failing to do so 338 will break Alpha, cause aggressive compilers to generate bad code, 339 and confuse people trying to read your code. 340 34111. Any lock acquired by an RCU callback must be acquired elsewhere 342 with softirq disabled, e.g., via spin_lock_irqsave(), 343 spin_lock_bh(), etc. Failing to disable softirq on a given 344 acquisition of that lock will result in deadlock as soon as 345 the RCU softirq handler happens to run your RCU callback while 346 interrupting that acquisition's critical section. 347 34812. RCU callbacks can be and are executed in parallel. In many cases, 349 the callback code simply wrappers around kfree(), so that this 350 is not an issue (or, more accurately, to the extent that it is 351 an issue, the memory-allocator locking handles it). However, 352 if the callbacks do manipulate a shared data structure, they 353 must use whatever locking or other synchronization is required 354 to safely access and/or modify that data structure. 355 356 Do not assume that RCU callbacks will be executed on the same 357 CPU that executed the corresponding call_rcu() or call_srcu(). 358 For example, if a given CPU goes offline while having an RCU 359 callback pending, then that RCU callback will execute on some 360 surviving CPU. (If this was not the case, a self-spawning RCU 361 callback would prevent the victim CPU from ever going offline.) 362 Furthermore, CPUs designated by rcu_nocbs= might well -always- 363 have their RCU callbacks executed on some other CPUs, in fact, 364 for some real-time workloads, this is the whole point of using 365 the rcu_nocbs= kernel boot parameter. 366 36713. Unlike other forms of RCU, it -is- permissible to block in an 368 SRCU read-side critical section (demarked by srcu_read_lock() 369 and srcu_read_unlock()), hence the "SRCU": "sleepable RCU". 370 Please note that if you don't need to sleep in read-side critical 371 sections, you should be using RCU rather than SRCU, because RCU 372 is almost always faster and easier to use than is SRCU. 373 374 Also unlike other forms of RCU, explicit initialization and 375 cleanup is required either at build time via DEFINE_SRCU() 376 or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct() 377 and cleanup_srcu_struct(). These last two are passed a 378 "struct srcu_struct" that defines the scope of a given 379 SRCU domain. Once initialized, the srcu_struct is passed 380 to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(), 381 synchronize_srcu_expedited(), and call_srcu(). A given 382 synchronize_srcu() waits only for SRCU read-side critical 383 sections governed by srcu_read_lock() and srcu_read_unlock() 384 calls that have been passed the same srcu_struct. This property 385 is what makes sleeping read-side critical sections tolerable -- 386 a given subsystem delays only its own updates, not those of other 387 subsystems using SRCU. Therefore, SRCU is less prone to OOM the 388 system than RCU would be if RCU's read-side critical sections 389 were permitted to sleep. 390 391 The ability to sleep in read-side critical sections does not 392 come for free. First, corresponding srcu_read_lock() and 393 srcu_read_unlock() calls must be passed the same srcu_struct. 394 Second, grace-period-detection overhead is amortized only 395 over those updates sharing a given srcu_struct, rather than 396 being globally amortized as they are for other forms of RCU. 397 Therefore, SRCU should be used in preference to rw_semaphore 398 only in extremely read-intensive situations, or in situations 399 requiring SRCU's read-side deadlock immunity or low read-side 400 realtime latency. You should also consider percpu_rw_semaphore 401 when you need lightweight readers. 402 403 SRCU's expedited primitive (synchronize_srcu_expedited()) 404 never sends IPIs to other CPUs, so it is easier on 405 real-time workloads than is synchronize_rcu_expedited(). 406 407 Note that rcu_assign_pointer() relates to SRCU just as it does to 408 other forms of RCU, but instead of rcu_dereference() you should 409 use srcu_dereference() in order to avoid lockdep splats. 410 41114. The whole point of call_rcu(), synchronize_rcu(), and friends 412 is to wait until all pre-existing readers have finished before 413 carrying out some otherwise-destructive operation. It is 414 therefore critically important to -first- remove any path 415 that readers can follow that could be affected by the 416 destructive operation, and -only- -then- invoke call_rcu(), 417 synchronize_rcu(), or friends. 418 419 Because these primitives only wait for pre-existing readers, it 420 is the caller's responsibility to guarantee that any subsequent 421 readers will execute safely. 422 42315. The various RCU read-side primitives do -not- necessarily contain 424 memory barriers. You should therefore plan for the CPU 425 and the compiler to freely reorder code into and out of RCU 426 read-side critical sections. It is the responsibility of the 427 RCU update-side primitives to deal with this. 428 429 For SRCU readers, you can use smp_mb__after_srcu_read_unlock() 430 immediately after an srcu_read_unlock() to get a full barrier. 431 43216. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the 433 __rcu sparse checks to validate your RCU code. These can help 434 find problems as follows: 435 436 CONFIG_PROVE_LOCKING: 437 check that accesses to RCU-protected data 438 structures are carried out under the proper RCU 439 read-side critical section, while holding the right 440 combination of locks, or whatever other conditions 441 are appropriate. 442 443 CONFIG_DEBUG_OBJECTS_RCU_HEAD: 444 check that you don't pass the 445 same object to call_rcu() (or friends) before an RCU 446 grace period has elapsed since the last time that you 447 passed that same object to call_rcu() (or friends). 448 449 __rcu sparse checks: 450 tag the pointer to the RCU-protected data 451 structure with __rcu, and sparse will warn you if you 452 access that pointer without the services of one of the 453 variants of rcu_dereference(). 454 455 These debugging aids can help you find problems that are 456 otherwise extremely difficult to spot. 457 45817. If you register a callback using call_rcu() or call_srcu(), and 459 pass in a function defined within a loadable module, then it in 460 necessary to wait for all pending callbacks to be invoked after 461 the last invocation and before unloading that module. Note that 462 it is absolutely -not- sufficient to wait for a grace period! 463 The current (say) synchronize_rcu() implementation is -not- 464 guaranteed to wait for callbacks registered on other CPUs. 465 Or even on the current CPU if that CPU recently went offline 466 and came back online. 467 468 You instead need to use one of the barrier functions: 469 470 - call_rcu() -> rcu_barrier() 471 - call_srcu() -> srcu_barrier() 472 473 However, these barrier functions are absolutely -not- guaranteed 474 to wait for a grace period. In fact, if there are no call_rcu() 475 callbacks waiting anywhere in the system, rcu_barrier() is within 476 its rights to return immediately. 477 478 So if you need to wait for both an RCU grace period and for 479 all pre-existing call_rcu() callbacks, you will need to execute 480 both rcu_barrier() and synchronize_rcu(), if necessary, using 481 something like workqueues to to execute them concurrently. 482 483 See rcubarrier.txt for more information. 484