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 can provide the same simplifications to certain types 36 of lockless algorithms that garbage 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 53 on large systems (for example, if the task is updating information 54 relating to itself that other tasks can read, there by definition 55 can be no bottleneck). Note that the definition of "large" has 56 changed significantly: Eight CPUs was "large" in the year 2000, 57 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 Explicit disabling of preemption (preempt_disable(), for example) 70 can serve as rcu_read_lock_sched(), but is less readable and 71 prevents lockdep from detecting locking issues. Acquiring a 72 raw spinlock also enters an RCU read-side critical section. 73 74 The guard(rcu)() and scoped_guard(rcu) primitives designate 75 the remainder of the current scope or the next statement, 76 respectively, as the RCU read-side critical section. Use of 77 these guards can be less error-prone than rcu_read_lock(), 78 rcu_read_unlock(), and friends. 79 80 Please note that you *cannot* rely on code known to be built 81 only in non-preemptible kernels. Such code can and will break, 82 especially in kernels built with CONFIG_PREEMPT_COUNT=y. 83 84 Letting RCU-protected pointers "leak" out of an RCU read-side 85 critical section is every bit as bad as letting them leak out 86 from under a lock. Unless, of course, you have arranged some 87 other means of protection, such as a lock or a reference count 88 *before* letting them out of the RCU read-side critical section. 89 903. Does the update code tolerate concurrent accesses? 91 92 The whole point of RCU is to permit readers to run without 93 any locks or atomic operations. This means that readers will 94 be running while updates are in progress. There are a number 95 of ways to handle this concurrency, depending on the situation: 96 97 a. Use the RCU variants of the list and hlist update 98 primitives to add, remove, and replace elements on 99 an RCU-protected list. Alternatively, use the other 100 RCU-protected data structures that have been added to 101 the Linux kernel. 102 103 This is almost always the best approach. 104 105 b. Proceed as in (a) above, but also maintain per-element 106 locks (that are acquired by both readers and writers) 107 that guard per-element state. Fields that the readers 108 refrain from accessing can be guarded by some other lock 109 acquired only by updaters, if desired. 110 111 This also works quite well. 112 113 c. Make updates appear atomic to readers. For example, 114 pointer updates to properly aligned fields will 115 appear atomic, as will individual atomic primitives. 116 Sequences of operations performed under a lock will *not* 117 appear to be atomic to RCU readers, nor will sequences 118 of multiple atomic primitives. One alternative is to 119 move multiple individual fields to a separate structure, 120 thus solving the multiple-field problem by imposing an 121 additional level of indirection. 122 123 This can work, but is starting to get a bit tricky. 124 125 d. Carefully order the updates and the reads so that readers 126 see valid data at all phases of the update. This is often 127 more difficult than it sounds, especially given modern 128 CPUs' tendency to reorder memory references. One must 129 usually liberally sprinkle memory-ordering operations 130 through the code, making it difficult to understand and 131 to test. Where it works, it is better to use things 132 like smp_store_release() and smp_load_acquire(), but in 133 some cases the smp_mb() full memory barrier is required. 134 135 As noted earlier, it is usually better to group the 136 changing data into a separate structure, so that the 137 change may be made to appear atomic by updating a pointer 138 to reference a new structure containing updated values. 139 1404. Weakly ordered CPUs pose special challenges. Almost all CPUs 141 are weakly ordered -- even x86 CPUs allow later loads to be 142 reordered to precede earlier stores. RCU code must take all of 143 the following measures to prevent memory-corruption problems: 144 145 a. Readers must maintain proper ordering of their memory 146 accesses. The rcu_dereference() primitive ensures that 147 the CPU picks up the pointer before it picks up the data 148 that the pointer points to. This really is necessary 149 on Alpha CPUs. 150 151 The rcu_dereference() primitive is also an excellent 152 documentation aid, letting the person reading the 153 code know exactly which pointers are protected by RCU. 154 Please note that compilers can also reorder code, and 155 they are becoming increasingly aggressive about doing 156 just that. The rcu_dereference() primitive therefore also 157 prevents destructive compiler optimizations. However, 158 with a bit of devious creativity, it is possible to 159 mishandle the return value from rcu_dereference(). 160 Please see rcu_dereference.rst for more information. 161 162 The rcu_dereference() primitive is used by the 163 various "_rcu()" list-traversal primitives, such 164 as the list_for_each_entry_rcu(). Note that it is 165 perfectly legal (if redundant) for update-side code to 166 use rcu_dereference() and the "_rcu()" list-traversal 167 primitives. This is particularly useful in code that 168 is common to readers and updaters. However, lockdep 169 will complain if you access rcu_dereference() outside 170 of an RCU read-side critical section. See lockdep.rst 171 to learn what to do about this. 172 173 Of course, neither rcu_dereference() nor the "_rcu()" 174 list-traversal primitives can substitute for a good 175 concurrency design coordinating among multiple updaters. 176 177 b. If the list macros are being used, the list_add_tail_rcu() 178 and list_add_rcu() primitives must be used in order 179 to prevent weakly ordered machines from misordering 180 structure initialization and pointer planting. 181 Similarly, if the hlist macros are being used, the 182 hlist_add_head_rcu() primitive is required. 183 184 c. If the list macros are being used, the list_del_rcu() 185 primitive must be used to keep list_del()'s pointer 186 poisoning from inflicting toxic effects on concurrent 187 readers. Similarly, if the hlist macros are being used, 188 the hlist_del_rcu() primitive is required. 189 190 The list_replace_rcu() and hlist_replace_rcu() primitives 191 may be used to replace an old structure with a new one 192 in their respective types of RCU-protected lists. 193 194 d. Rules similar to (4b) and (4c) apply to the "hlist_nulls" 195 type of RCU-protected linked lists. 196 197 e. Updates must ensure that initialization of a given 198 structure happens before pointers to that structure are 199 publicized. Use the rcu_assign_pointer() primitive 200 when publicizing a pointer to a structure that can 201 be traversed by an RCU read-side critical section. 202 2035. If any of call_rcu(), call_srcu(), call_rcu_tasks(), or 204 call_rcu_tasks_trace() is used, the callback function may be 205 invoked from softirq context, and in any case with bottom halves 206 disabled. In particular, this callback function cannot block. 207 If you need the callback to block, run that code in a workqueue 208 handler scheduled from the callback. The queue_rcu_work() 209 function does this for you in the case of call_rcu(). 210 2116. Since synchronize_rcu() can block, it cannot be called 212 from any sort of irq context. The same rule applies 213 for synchronize_srcu(), synchronize_rcu_expedited(), 214 synchronize_srcu_expedited(), synchronize_rcu_tasks(), 215 synchronize_rcu_tasks_rude(), and synchronize_rcu_tasks_trace(). 216 217 The expedited forms of these primitives have the same semantics 218 as the non-expedited forms, but expediting is more CPU intensive. 219 Use of the expedited primitives should be restricted to rare 220 configuration-change operations that would not normally be 221 undertaken while a real-time workload is running. Note that 222 IPI-sensitive real-time workloads can use the rcupdate.rcu_normal 223 kernel boot parameter to completely disable expedited grace 224 periods, though this might have performance implications. 225 226 In particular, if you find yourself invoking one of the expedited 227 primitives repeatedly in a loop, please do everyone a favor: 228 Restructure your code so that it batches the updates, allowing 229 a single non-expedited primitive to cover the entire batch. 230 This will very likely be faster than the loop containing the 231 expedited primitive, and will be much much easier on the rest 232 of the system, especially to real-time workloads running on the 233 rest of the system. Alternatively, instead use asynchronous 234 primitives such as call_rcu(). 235 2367. As of v4.20, a given kernel implements only one RCU flavor, which 237 is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y. 238 If the updater uses call_rcu() or synchronize_rcu(), then 239 the corresponding readers may use: (1) rcu_read_lock() and 240 rcu_read_unlock(), (2) any pair of primitives that disables 241 and re-enables softirq, for example, rcu_read_lock_bh() and 242 rcu_read_unlock_bh(), or (3) any pair of primitives that disables 243 and re-enables preemption, for example, rcu_read_lock_sched() and 244 rcu_read_unlock_sched(). If the updater uses synchronize_srcu() 245 or call_srcu(), then the corresponding readers must use 246 srcu_read_lock() and srcu_read_unlock(), and with the same 247 srcu_struct. The rules for the expedited RCU grace-period-wait 248 primitives are the same as for their non-expedited counterparts. 249 250 Similarly, it is necessary to correctly use the RCU Tasks flavors: 251 252 a. If the updater uses synchronize_rcu_tasks() or 253 call_rcu_tasks(), then the readers must refrain from 254 executing voluntary context switches, that is, from 255 blocking. 256 257 b. If the updater uses call_rcu_tasks_trace() 258 or synchronize_rcu_tasks_trace(), then the 259 corresponding readers must use rcu_read_lock_trace() 260 and rcu_read_unlock_trace(). 261 262 c. If an updater uses synchronize_rcu_tasks_rude(), 263 then the corresponding readers must use anything that 264 disables preemption, for example, preempt_disable() 265 and preempt_enable(). 266 267 Mixing things up will result in confusion and broken kernels, and 268 has even resulted in an exploitable security issue. Therefore, 269 when using non-obvious pairs of primitives, commenting is 270 of course a must. One example of non-obvious pairing is 271 the XDP feature in networking, which calls BPF programs from 272 network-driver NAPI (softirq) context. BPF relies heavily on RCU 273 protection for its data structures, but because the BPF program 274 invocation happens entirely within a single local_bh_disable() 275 section in a NAPI poll cycle, this usage is safe. The reason 276 that this usage is safe is that readers can use anything that 277 disables BH when updaters use call_rcu() or synchronize_rcu(). 278 2798. Although synchronize_rcu() is slower than is call_rcu(), 280 it usually results in simpler code. So, unless update 281 performance is critically important, the updaters cannot block, 282 or the latency of synchronize_rcu() is visible from userspace, 283 synchronize_rcu() should be used in preference to call_rcu(). 284 Furthermore, kfree_rcu() and kvfree_rcu() usually result 285 in even simpler code than does synchronize_rcu() without 286 synchronize_rcu()'s multi-millisecond latency. So please take 287 advantage of kfree_rcu()'s and kvfree_rcu()'s "fire and forget" 288 memory-freeing capabilities where it applies. 289 290 An especially important property of the synchronize_rcu() 291 primitive is that it automatically self-limits: if grace periods 292 are delayed for whatever reason, then the synchronize_rcu() 293 primitive will correspondingly delay updates. In contrast, 294 code using call_rcu() should explicitly limit update rate in 295 cases where grace periods are delayed, as failing to do so can 296 result in excessive realtime latencies or even OOM conditions. 297 298 Ways of gaining this self-limiting property when using call_rcu(), 299 kfree_rcu(), or kvfree_rcu() include: 300 301 a. Keeping a count of the number of data-structure elements 302 used by the RCU-protected data structure, including 303 those waiting for a grace period to elapse. Enforce a 304 limit on this number, stalling updates as needed to allow 305 previously deferred frees to complete. Alternatively, 306 limit only the number awaiting deferred free rather than 307 the total number of elements. 308 309 One way to stall the updates is to acquire the update-side 310 mutex. (Don't try this with a spinlock -- other CPUs 311 spinning on the lock could prevent the grace period 312 from ever ending.) Another way to stall the updates 313 is for the updates to use a wrapper function around 314 the memory allocator, so that this wrapper function 315 simulates OOM when there is too much memory awaiting an 316 RCU grace period. There are of course many other 317 variations on this theme. 318 319 b. Limiting update rate. For example, if updates occur only 320 once per hour, then no explicit rate limiting is 321 required, unless your system is already badly broken. 322 Older versions of the dcache subsystem take this approach, 323 guarding updates with a global lock, limiting their rate. 324 325 c. Trusted update -- if updates can only be done manually by 326 superuser or some other trusted user, then it might not 327 be necessary to automatically limit them. The theory 328 here is that superuser already has lots of ways to crash 329 the machine. 330 331 d. Periodically invoke rcu_barrier(), permitting a limited 332 number of updates per grace period. 333 334 The same cautions apply to call_srcu(), call_rcu_tasks(), and 335 call_rcu_tasks_trace(). This is why there is an srcu_barrier(), 336 rcu_barrier_tasks(), and rcu_barrier_tasks_trace(), respectively. 337 338 Note that although these primitives do take action to avoid 339 memory exhaustion when any given CPU has too many callbacks, 340 a determined user or administrator can still exhaust memory. 341 This is especially the case if a system with a large number of 342 CPUs has been configured to offload all of its RCU callbacks onto 343 a single CPU, or if the system has relatively little free memory. 344 3459. All RCU list-traversal primitives, which include 346 rcu_dereference(), list_for_each_entry_rcu(), and 347 list_for_each_safe_rcu(), must be either within an RCU read-side 348 critical section or must be protected by appropriate update-side 349 locks. RCU read-side critical sections are delimited by 350 rcu_read_lock() and rcu_read_unlock(), or by similar primitives 351 such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which 352 case the matching rcu_dereference() primitive must be used in 353 order to keep lockdep happy, in this case, rcu_dereference_bh(). 354 355 The reason that it is permissible to use RCU list-traversal 356 primitives when the update-side lock is held is that doing so 357 can be quite helpful in reducing code bloat when common code is 358 shared between readers and updaters. Additional primitives 359 are provided for this case, as discussed in lockdep.rst. 360 361 One exception to this rule is when data is only ever added to 362 the linked data structure, and is never removed during any 363 time that readers might be accessing that structure. In such 364 cases, READ_ONCE() may be used in place of rcu_dereference() 365 and the read-side markers (rcu_read_lock() and rcu_read_unlock(), 366 for example) may be omitted. 367 36810. Conversely, if you are in an RCU read-side critical section, 369 and you don't hold the appropriate update-side lock, you *must* 370 use the "_rcu()" variants of the list macros. Failing to do so 371 will break Alpha, cause aggressive compilers to generate bad code, 372 and confuse people trying to understand your code. 373 37411. Any lock acquired by an RCU callback must be acquired elsewhere 375 with softirq disabled, e.g., via spin_lock_bh(). Failing to 376 disable softirq on a given acquisition of that lock will result 377 in deadlock as soon as the RCU softirq handler happens to run 378 your RCU callback while interrupting that acquisition's critical 379 section. 380 38112. RCU callbacks can be and are executed in parallel. In many cases, 382 the callback code simply wrappers around kfree(), so that this 383 is not an issue (or, more accurately, to the extent that it is 384 an issue, the memory-allocator locking handles it). However, 385 if the callbacks do manipulate a shared data structure, they 386 must use whatever locking or other synchronization is required 387 to safely access and/or modify that data structure. 388 389 Do not assume that RCU callbacks will be executed on the same 390 CPU that executed the corresponding call_rcu(), call_srcu(), 391 call_rcu_tasks(), or call_rcu_tasks_trace(). For example, if 392 a given CPU goes offline while having an RCU callback pending, 393 then that RCU callback will execute on some surviving CPU. 394 (If this was not the case, a self-spawning RCU callback would 395 prevent the victim CPU from ever going offline.) Furthermore, 396 CPUs designated by rcu_nocbs= might well *always* have their 397 RCU callbacks executed on some other CPUs, in fact, for some 398 real-time workloads, this is the whole point of using the 399 rcu_nocbs= kernel boot parameter. 400 401 In addition, do not assume that callbacks queued in a given order 402 will be invoked in that order, even if they all are queued on the 403 same CPU. Furthermore, do not assume that same-CPU callbacks will 404 be invoked serially. For example, in recent kernels, CPUs can be 405 switched between offloaded and de-offloaded callback invocation, 406 and while a given CPU is undergoing such a switch, its callbacks 407 might be concurrently invoked by that CPU's softirq handler and 408 that CPU's rcuo kthread. At such times, that CPU's callbacks 409 might be executed both concurrently and out of order. 410 41113. Unlike most flavors of RCU, it *is* permissible to block in an 412 SRCU read-side critical section (demarked by srcu_read_lock() 413 and srcu_read_unlock()), hence the "SRCU": "sleepable RCU". 414 As with RCU, guard(srcu)() and scoped_guard(srcu) forms are 415 available, and often provide greater ease of use. Please note 416 that if you don't need to sleep in read-side critical sections, 417 you should be using RCU rather than SRCU, because RCU is almost 418 always faster and easier to use than is SRCU. 419 420 Also unlike other forms of RCU, explicit initialization and 421 cleanup is required either at build time via DEFINE_SRCU() 422 or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct() 423 and cleanup_srcu_struct(). These last two are passed a 424 "struct srcu_struct" that defines the scope of a given 425 SRCU domain. Once initialized, the srcu_struct is passed 426 to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(), 427 synchronize_srcu_expedited(), and call_srcu(). A given 428 synchronize_srcu() waits only for SRCU read-side critical 429 sections governed by srcu_read_lock() and srcu_read_unlock() 430 calls that have been passed the same srcu_struct. This property 431 is what makes sleeping read-side critical sections tolerable -- 432 a given subsystem delays only its own updates, not those of other 433 subsystems using SRCU. Therefore, SRCU is less prone to OOM the 434 system than RCU would be if RCU's read-side critical sections 435 were permitted to sleep. 436 437 The ability to sleep in read-side critical sections does not 438 come for free. First, corresponding srcu_read_lock() and 439 srcu_read_unlock() calls must be passed the same srcu_struct. 440 Second, grace-period-detection overhead is amortized only 441 over those updates sharing a given srcu_struct, rather than 442 being globally amortized as they are for other forms of RCU. 443 Therefore, SRCU should be used in preference to rw_semaphore 444 only in extremely read-intensive situations, or in situations 445 requiring SRCU's read-side deadlock immunity or low read-side 446 realtime latency. You should also consider percpu_rw_semaphore 447 when you need lightweight readers. 448 449 SRCU's expedited primitive (synchronize_srcu_expedited()) 450 never sends IPIs to other CPUs, so it is easier on 451 real-time workloads than is synchronize_rcu_expedited(). 452 453 It is also permissible to sleep in RCU Tasks Trace read-side 454 critical section, which are delimited by rcu_read_lock_trace() 455 and rcu_read_unlock_trace(). However, this is a specialized 456 flavor of RCU, and you should not use it without first checking 457 with its current users. In most cases, you should instead 458 use SRCU. As with RCU and SRCU, guard(rcu_tasks_trace)() and 459 scoped_guard(rcu_tasks_trace) are available, and often provide 460 greater ease of use. 461 462 Note that rcu_assign_pointer() relates to SRCU just as it does to 463 other forms of RCU, but instead of rcu_dereference() you should 464 use srcu_dereference() in order to avoid lockdep splats. 465 46614. The whole point of call_rcu(), synchronize_rcu(), and friends 467 is to wait until all pre-existing readers have finished before 468 carrying out some otherwise-destructive operation. It is 469 therefore critically important to *first* remove any path 470 that readers can follow that could be affected by the 471 destructive operation, and *only then* invoke call_rcu(), 472 synchronize_rcu(), or friends. 473 474 Because these primitives only wait for pre-existing readers, it 475 is the caller's responsibility to guarantee that any subsequent 476 readers will execute safely. 477 47815. The various RCU read-side primitives do *not* necessarily contain 479 memory barriers. You should therefore plan for the CPU 480 and the compiler to freely reorder code into and out of RCU 481 read-side critical sections. It is the responsibility of the 482 RCU update-side primitives to deal with this. 483 484 For SRCU readers, you can use smp_mb__after_srcu_read_unlock() 485 immediately after an srcu_read_unlock() to get a full barrier. 486 48716. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the 488 __rcu sparse checks to validate your RCU code. These can help 489 find problems as follows: 490 491 CONFIG_PROVE_LOCKING: 492 check that accesses to RCU-protected data structures 493 are carried out under the proper RCU read-side critical 494 section, while holding the right combination of locks, 495 or whatever other conditions are appropriate. 496 497 CONFIG_DEBUG_OBJECTS_RCU_HEAD: 498 check that you don't pass the same object to call_rcu() 499 (or friends) before an RCU grace period has elapsed 500 since the last time that you passed that same object to 501 call_rcu() (or friends). 502 503 CONFIG_RCU_STRICT_GRACE_PERIOD: 504 combine with KASAN to check for pointers leaked out 505 of RCU read-side critical sections. This Kconfig 506 option is tough on both performance and scalability, 507 and so is limited to four-CPU systems. 508 509 __rcu sparse checks: 510 tag the pointer to the RCU-protected data structure 511 with __rcu, and sparse will warn you if you access that 512 pointer without the services of one of the variants 513 of rcu_dereference(). 514 515 These debugging aids can help you find problems that are 516 otherwise extremely difficult to spot. 517 51817. If you pass a callback function defined within a module 519 to one of call_rcu(), call_srcu(), call_rcu_tasks(), or 520 call_rcu_tasks_trace(), then it is necessary to wait for all 521 pending callbacks to be invoked before unloading that module. 522 Note that it is absolutely *not* sufficient to wait for a grace 523 period! For example, synchronize_rcu() implementation is *not* 524 guaranteed to wait for callbacks registered on other CPUs via 525 call_rcu(). Or even on the current CPU if that CPU recently 526 went offline and came back online. 527 528 You instead need to use one of the barrier functions: 529 530 - call_rcu() -> rcu_barrier() 531 - call_srcu() -> srcu_barrier() 532 - call_rcu_tasks() -> rcu_barrier_tasks() 533 - call_rcu_tasks_trace() -> rcu_barrier_tasks_trace() 534 535 However, these barrier functions are absolutely *not* guaranteed 536 to wait for a grace period. For example, if there are no 537 call_rcu() callbacks queued anywhere in the system, rcu_barrier() 538 can and will return immediately. 539 540 So if you need to wait for both a grace period and for all 541 pre-existing callbacks, you will need to invoke both functions, 542 with the pair depending on the flavor of RCU: 543 544 - Either synchronize_rcu() or synchronize_rcu_expedited(), 545 together with rcu_barrier() 546 - Either synchronize_srcu() or synchronize_srcu_expedited(), 547 together with and srcu_barrier() 548 - synchronize_rcu_tasks() and rcu_barrier_tasks() 549 - synchronize_tasks_trace() and rcu_barrier_tasks_trace() 550 551 If necessary, you can use something like workqueues to execute 552 the requisite pair of functions concurrently. 553 554 See rcubarrier.rst for more information. 555