Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
1 What is RCU? 2 3 RCU is a synchronization mechanism that was added to the Linux kernel 4 during the 2.5 development effort that is optimized for read-mostly 5 situations. Although RCU is actually quite simple once you understand it, 6 getting there can sometimes be a challenge. Part of the problem is that 7 most of the past descriptions of RCU have been written with the mistaken 8 assumption that there is "one true way" to describe RCU. Instead, 9 the experience has been that different people must take different paths 10 to arrive at an understanding of RCU. This document provides several 11 different paths, as follows: 12 13 1. RCU OVERVIEW 14 2. WHAT IS RCU'S CORE API? 15 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? 16 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? 17 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? 18 6. ANALOGY WITH READER-WRITER LOCKING 19 7. FULL LIST OF RCU APIs 20 8. ANSWERS TO QUICK QUIZZES 21 22 People who prefer starting with a conceptual overview should focus on 23 Section 1, though most readers will profit by reading this section at 24 some point. People who prefer to start with an API that they can then 25 experiment with should focus on Section 2. People who prefer to start 26 with example uses should focus on Sections 3 and 4. People who need to 27 understand the RCU implementation should focus on Section 5, then dive 28 into the kernel source code. People who reason best by analogy should 29 focus on Section 6. Section 7 serves as an index to the docbook API 30 documentation, and Section 8 is the traditional answer key. 31 32 So, start with the section that makes the most sense to you and your 33 preferred method of learning. If you need to know everything about 34 everything, feel free to read the whole thing -- but if you are really 35 that type of person, you have perused the source code and will therefore 36 never need this document anyway. ;-) 37 38 39 1. RCU OVERVIEW 40 41 The basic idea behind RCU is to split updates into "removal" and 42 "reclamation" phases. The removal phase removes references to data items 43 within a data structure (possibly by replacing them with references to 44 new versions of these data items), and can run concurrently with readers. 45 The reason that it is safe to run the removal phase concurrently with 46 readers is the semantics of modern CPUs guarantee that readers will see 47 either the old or the new version of the data structure rather than a 48 partially updated reference. The reclamation phase does the work of reclaiming 49 (e.g., freeing) the data items removed from the data structure during the 50 removal phase. Because reclaiming data items can disrupt any readers 51 concurrently referencing those data items, the reclamation phase must 52 not start until readers no longer hold references to those data items. 53 54 Splitting the update into removal and reclamation phases permits the 55 updater to perform the removal phase immediately, and to defer the 56 reclamation phase until all readers active during the removal phase have 57 completed, either by blocking until they finish or by registering a 58 callback that is invoked after they finish. Only readers that are active 59 during the removal phase need be considered, because any reader starting 60 after the removal phase will be unable to gain a reference to the removed 61 data items, and therefore cannot be disrupted by the reclamation phase. 62 63 So the typical RCU update sequence goes something like the following: 64 65 a. Remove pointers to a data structure, so that subsequent 66 readers cannot gain a reference to it. 67 68 b. Wait for all previous readers to complete their RCU read-side 69 critical sections. 70 71 c. At this point, there cannot be any readers who hold references 72 to the data structure, so it now may safely be reclaimed 73 (e.g., kfree()d). 74 75 Step (b) above is the key idea underlying RCU's deferred destruction. 76 The ability to wait until all readers are done allows RCU readers to 77 use much lighter-weight synchronization, in some cases, absolutely no 78 synchronization at all. In contrast, in more conventional lock-based 79 schemes, readers must use heavy-weight synchronization in order to 80 prevent an updater from deleting the data structure out from under them. 81 This is because lock-based updaters typically update data items in place, 82 and must therefore exclude readers. In contrast, RCU-based updaters 83 typically take advantage of the fact that writes to single aligned 84 pointers are atomic on modern CPUs, allowing atomic insertion, removal, 85 and replacement of data items in a linked structure without disrupting 86 readers. Concurrent RCU readers can then continue accessing the old 87 versions, and can dispense with the atomic operations, memory barriers, 88 and communications cache misses that are so expensive on present-day 89 SMP computer systems, even in absence of lock contention. 90 91 In the three-step procedure shown above, the updater is performing both 92 the removal and the reclamation step, but it is often helpful for an 93 entirely different thread to do the reclamation, as is in fact the case 94 in the Linux kernel's directory-entry cache (dcache). Even if the same 95 thread performs both the update step (step (a) above) and the reclamation 96 step (step (c) above), it is often helpful to think of them separately. 97 For example, RCU readers and updaters need not communicate at all, 98 but RCU provides implicit low-overhead communication between readers 99 and reclaimers, namely, in step (b) above. 100 101 So how the heck can a reclaimer tell when a reader is done, given 102 that readers are not doing any sort of synchronization operations??? 103 Read on to learn about how RCU's API makes this easy. 104 105 106 2. WHAT IS RCU'S CORE API? 107 108 The core RCU API is quite small: 109 110 a. rcu_read_lock() 111 b. rcu_read_unlock() 112 c. synchronize_rcu() / call_rcu() 113 d. rcu_assign_pointer() 114 e. rcu_dereference() 115 116 There are many other members of the RCU API, but the rest can be 117 expressed in terms of these five, though most implementations instead 118 express synchronize_rcu() in terms of the call_rcu() callback API. 119 120 The five core RCU APIs are described below, the other 18 will be enumerated 121 later. See the kernel docbook documentation for more info, or look directly 122 at the function header comments. 123 124 rcu_read_lock() 125 126 void rcu_read_lock(void); 127 128 Used by a reader to inform the reclaimer that the reader is 129 entering an RCU read-side critical section. It is illegal 130 to block while in an RCU read-side critical section, though 131 kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side 132 critical sections. Any RCU-protected data structure accessed 133 during an RCU read-side critical section is guaranteed to remain 134 unreclaimed for the full duration of that critical section. 135 Reference counts may be used in conjunction with RCU to maintain 136 longer-term references to data structures. 137 138 rcu_read_unlock() 139 140 void rcu_read_unlock(void); 141 142 Used by a reader to inform the reclaimer that the reader is 143 exiting an RCU read-side critical section. Note that RCU 144 read-side critical sections may be nested and/or overlapping. 145 146 synchronize_rcu() 147 148 void synchronize_rcu(void); 149 150 Marks the end of updater code and the beginning of reclaimer 151 code. It does this by blocking until all pre-existing RCU 152 read-side critical sections on all CPUs have completed. 153 Note that synchronize_rcu() will -not- necessarily wait for 154 any subsequent RCU read-side critical sections to complete. 155 For example, consider the following sequence of events: 156 157 CPU 0 CPU 1 CPU 2 158 ----------------- ------------------------- --------------- 159 1. rcu_read_lock() 160 2. enters synchronize_rcu() 161 3. rcu_read_lock() 162 4. rcu_read_unlock() 163 5. exits synchronize_rcu() 164 6. rcu_read_unlock() 165 166 To reiterate, synchronize_rcu() waits only for ongoing RCU 167 read-side critical sections to complete, not necessarily for 168 any that begin after synchronize_rcu() is invoked. 169 170 Of course, synchronize_rcu() does not necessarily return 171 -immediately- after the last pre-existing RCU read-side critical 172 section completes. For one thing, there might well be scheduling 173 delays. For another thing, many RCU implementations process 174 requests in batches in order to improve efficiencies, which can 175 further delay synchronize_rcu(). 176 177 Since synchronize_rcu() is the API that must figure out when 178 readers are done, its implementation is key to RCU. For RCU 179 to be useful in all but the most read-intensive situations, 180 synchronize_rcu()'s overhead must also be quite small. 181 182 The call_rcu() API is a callback form of synchronize_rcu(), 183 and is described in more detail in a later section. Instead of 184 blocking, it registers a function and argument which are invoked 185 after all ongoing RCU read-side critical sections have completed. 186 This callback variant is particularly useful in situations where 187 it is illegal to block or where update-side performance is 188 critically important. 189 190 However, the call_rcu() API should not be used lightly, as use 191 of the synchronize_rcu() API generally results in simpler code. 192 In addition, the synchronize_rcu() API has the nice property 193 of automatically limiting update rate should grace periods 194 be delayed. This property results in system resilience in face 195 of denial-of-service attacks. Code using call_rcu() should limit 196 update rate in order to gain this same sort of resilience. See 197 checklist.txt for some approaches to limiting the update rate. 198 199 rcu_assign_pointer() 200 201 typeof(p) rcu_assign_pointer(p, typeof(p) v); 202 203 Yes, rcu_assign_pointer() -is- implemented as a macro, though it 204 would be cool to be able to declare a function in this manner. 205 (Compiler experts will no doubt disagree.) 206 207 The updater uses this function to assign a new value to an 208 RCU-protected pointer, in order to safely communicate the change 209 in value from the updater to the reader. This function returns 210 the new value, and also executes any memory-barrier instructions 211 required for a given CPU architecture. 212 213 Perhaps just as important, it serves to document (1) which 214 pointers are protected by RCU and (2) the point at which a 215 given structure becomes accessible to other CPUs. That said, 216 rcu_assign_pointer() is most frequently used indirectly, via 217 the _rcu list-manipulation primitives such as list_add_rcu(). 218 219 rcu_dereference() 220 221 typeof(p) rcu_dereference(p); 222 223 Like rcu_assign_pointer(), rcu_dereference() must be implemented 224 as a macro. 225 226 The reader uses rcu_dereference() to fetch an RCU-protected 227 pointer, which returns a value that may then be safely 228 dereferenced. Note that rcu_deference() does not actually 229 dereference the pointer, instead, it protects the pointer for 230 later dereferencing. It also executes any needed memory-barrier 231 instructions for a given CPU architecture. Currently, only Alpha 232 needs memory barriers within rcu_dereference() -- on other CPUs, 233 it compiles to nothing, not even a compiler directive. 234 235 Common coding practice uses rcu_dereference() to copy an 236 RCU-protected pointer to a local variable, then dereferences 237 this local variable, for example as follows: 238 239 p = rcu_dereference(head.next); 240 return p->data; 241 242 However, in this case, one could just as easily combine these 243 into one statement: 244 245 return rcu_dereference(head.next)->data; 246 247 If you are going to be fetching multiple fields from the 248 RCU-protected structure, using the local variable is of 249 course preferred. Repeated rcu_dereference() calls look 250 ugly and incur unnecessary overhead on Alpha CPUs. 251 252 Note that the value returned by rcu_dereference() is valid 253 only within the enclosing RCU read-side critical section. 254 For example, the following is -not- legal: 255 256 rcu_read_lock(); 257 p = rcu_dereference(head.next); 258 rcu_read_unlock(); 259 x = p->address; 260 rcu_read_lock(); 261 y = p->data; 262 rcu_read_unlock(); 263 264 Holding a reference from one RCU read-side critical section 265 to another is just as illegal as holding a reference from 266 one lock-based critical section to another! Similarly, 267 using a reference outside of the critical section in which 268 it was acquired is just as illegal as doing so with normal 269 locking. 270 271 As with rcu_assign_pointer(), an important function of 272 rcu_dereference() is to document which pointers are protected by 273 RCU, in particular, flagging a pointer that is subject to changing 274 at any time, including immediately after the rcu_dereference(). 275 And, again like rcu_assign_pointer(), rcu_dereference() is 276 typically used indirectly, via the _rcu list-manipulation 277 primitives, such as list_for_each_entry_rcu(). 278 279 The following diagram shows how each API communicates among the 280 reader, updater, and reclaimer. 281 282 283 rcu_assign_pointer() 284 +--------+ 285 +---------------------->| reader |---------+ 286 | +--------+ | 287 | | | 288 | | | Protect: 289 | | | rcu_read_lock() 290 | | | rcu_read_unlock() 291 | rcu_dereference() | | 292 +---------+ | | 293 | updater |<---------------------+ | 294 +---------+ V 295 | +-----------+ 296 +----------------------------------->| reclaimer | 297 +-----------+ 298 Defer: 299 synchronize_rcu() & call_rcu() 300 301 302 The RCU infrastructure observes the time sequence of rcu_read_lock(), 303 rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in 304 order to determine when (1) synchronize_rcu() invocations may return 305 to their callers and (2) call_rcu() callbacks may be invoked. Efficient 306 implementations of the RCU infrastructure make heavy use of batching in 307 order to amortize their overhead over many uses of the corresponding APIs. 308 309 There are no fewer than three RCU mechanisms in the Linux kernel; the 310 diagram above shows the first one, which is by far the most commonly used. 311 The rcu_dereference() and rcu_assign_pointer() primitives are used for 312 all three mechanisms, but different defer and protect primitives are 313 used as follows: 314 315 Defer Protect 316 317 a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock() 318 call_rcu() 319 320 b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh() 321 322 c. synchronize_sched() preempt_disable() / preempt_enable() 323 local_irq_save() / local_irq_restore() 324 hardirq enter / hardirq exit 325 NMI enter / NMI exit 326 327 These three mechanisms are used as follows: 328 329 a. RCU applied to normal data structures. 330 331 b. RCU applied to networking data structures that may be subjected 332 to remote denial-of-service attacks. 333 334 c. RCU applied to scheduler and interrupt/NMI-handler tasks. 335 336 Again, most uses will be of (a). The (b) and (c) cases are important 337 for specialized uses, but are relatively uncommon. 338 339 340 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? 341 342 This section shows a simple use of the core RCU API to protect a 343 global pointer to a dynamically allocated structure. More-typical 344 uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. 345 346 struct foo { 347 int a; 348 char b; 349 long c; 350 }; 351 DEFINE_SPINLOCK(foo_mutex); 352 353 struct foo *gbl_foo; 354 355 /* 356 * Create a new struct foo that is the same as the one currently 357 * pointed to by gbl_foo, except that field "a" is replaced 358 * with "new_a". Points gbl_foo to the new structure, and 359 * frees up the old structure after a grace period. 360 * 361 * Uses rcu_assign_pointer() to ensure that concurrent readers 362 * see the initialized version of the new structure. 363 * 364 * Uses synchronize_rcu() to ensure that any readers that might 365 * have references to the old structure complete before freeing 366 * the old structure. 367 */ 368 void foo_update_a(int new_a) 369 { 370 struct foo *new_fp; 371 struct foo *old_fp; 372 373 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL); 374 spin_lock(&foo_mutex); 375 old_fp = gbl_foo; 376 *new_fp = *old_fp; 377 new_fp->a = new_a; 378 rcu_assign_pointer(gbl_foo, new_fp); 379 spin_unlock(&foo_mutex); 380 synchronize_rcu(); 381 kfree(old_fp); 382 } 383 384 /* 385 * Return the value of field "a" of the current gbl_foo 386 * structure. Use rcu_read_lock() and rcu_read_unlock() 387 * to ensure that the structure does not get deleted out 388 * from under us, and use rcu_dereference() to ensure that 389 * we see the initialized version of the structure (important 390 * for DEC Alpha and for people reading the code). 391 */ 392 int foo_get_a(void) 393 { 394 int retval; 395 396 rcu_read_lock(); 397 retval = rcu_dereference(gbl_foo)->a; 398 rcu_read_unlock(); 399 return retval; 400 } 401 402 So, to sum up: 403 404 o Use rcu_read_lock() and rcu_read_unlock() to guard RCU 405 read-side critical sections. 406 407 o Within an RCU read-side critical section, use rcu_dereference() 408 to dereference RCU-protected pointers. 409 410 o Use some solid scheme (such as locks or semaphores) to 411 keep concurrent updates from interfering with each other. 412 413 o Use rcu_assign_pointer() to update an RCU-protected pointer. 414 This primitive protects concurrent readers from the updater, 415 -not- concurrent updates from each other! You therefore still 416 need to use locking (or something similar) to keep concurrent 417 rcu_assign_pointer() primitives from interfering with each other. 418 419 o Use synchronize_rcu() -after- removing a data element from an 420 RCU-protected data structure, but -before- reclaiming/freeing 421 the data element, in order to wait for the completion of all 422 RCU read-side critical sections that might be referencing that 423 data item. 424 425 See checklist.txt for additional rules to follow when using RCU. 426 And again, more-typical uses of RCU may be found in listRCU.txt, 427 arrayRCU.txt, and NMI-RCU.txt. 428 429 430 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? 431 432 In the example above, foo_update_a() blocks until a grace period elapses. 433 This is quite simple, but in some cases one cannot afford to wait so 434 long -- there might be other high-priority work to be done. 435 436 In such cases, one uses call_rcu() rather than synchronize_rcu(). 437 The call_rcu() API is as follows: 438 439 void call_rcu(struct rcu_head * head, 440 void (*func)(struct rcu_head *head)); 441 442 This function invokes func(head) after a grace period has elapsed. 443 This invocation might happen from either softirq or process context, 444 so the function is not permitted to block. The foo struct needs to 445 have an rcu_head structure added, perhaps as follows: 446 447 struct foo { 448 int a; 449 char b; 450 long c; 451 struct rcu_head rcu; 452 }; 453 454 The foo_update_a() function might then be written as follows: 455 456 /* 457 * Create a new struct foo that is the same as the one currently 458 * pointed to by gbl_foo, except that field "a" is replaced 459 * with "new_a". Points gbl_foo to the new structure, and 460 * frees up the old structure after a grace period. 461 * 462 * Uses rcu_assign_pointer() to ensure that concurrent readers 463 * see the initialized version of the new structure. 464 * 465 * Uses call_rcu() to ensure that any readers that might have 466 * references to the old structure complete before freeing the 467 * old structure. 468 */ 469 void foo_update_a(int new_a) 470 { 471 struct foo *new_fp; 472 struct foo *old_fp; 473 474 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL); 475 spin_lock(&foo_mutex); 476 old_fp = gbl_foo; 477 *new_fp = *old_fp; 478 new_fp->a = new_a; 479 rcu_assign_pointer(gbl_foo, new_fp); 480 spin_unlock(&foo_mutex); 481 call_rcu(&old_fp->rcu, foo_reclaim); 482 } 483 484 The foo_reclaim() function might appear as follows: 485 486 void foo_reclaim(struct rcu_head *rp) 487 { 488 struct foo *fp = container_of(rp, struct foo, rcu); 489 490 kfree(fp); 491 } 492 493 The container_of() primitive is a macro that, given a pointer into a 494 struct, the type of the struct, and the pointed-to field within the 495 struct, returns a pointer to the beginning of the struct. 496 497 The use of call_rcu() permits the caller of foo_update_a() to 498 immediately regain control, without needing to worry further about the 499 old version of the newly updated element. It also clearly shows the 500 RCU distinction between updater, namely foo_update_a(), and reclaimer, 501 namely foo_reclaim(). 502 503 The summary of advice is the same as for the previous section, except 504 that we are now using call_rcu() rather than synchronize_rcu(): 505 506 o Use call_rcu() -after- removing a data element from an 507 RCU-protected data structure in order to register a callback 508 function that will be invoked after the completion of all RCU 509 read-side critical sections that might be referencing that 510 data item. 511 512 Again, see checklist.txt for additional rules governing the use of RCU. 513 514 515 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? 516 517 One of the nice things about RCU is that it has extremely simple "toy" 518 implementations that are a good first step towards understanding the 519 production-quality implementations in the Linux kernel. This section 520 presents two such "toy" implementations of RCU, one that is implemented 521 in terms of familiar locking primitives, and another that more closely 522 resembles "classic" RCU. Both are way too simple for real-world use, 523 lacking both functionality and performance. However, they are useful 524 in getting a feel for how RCU works. See kernel/rcupdate.c for a 525 production-quality implementation, and see: 526 527 http://www.rdrop.com/users/paulmck/RCU 528 529 for papers describing the Linux kernel RCU implementation. The OLS'01 530 and OLS'02 papers are a good introduction, and the dissertation provides 531 more details on the current implementation as of early 2004. 532 533 534 5A. "TOY" IMPLEMENTATION #1: LOCKING 535 536 This section presents a "toy" RCU implementation that is based on 537 familiar locking primitives. Its overhead makes it a non-starter for 538 real-life use, as does its lack of scalability. It is also unsuitable 539 for realtime use, since it allows scheduling latency to "bleed" from 540 one read-side critical section to another. 541 542 However, it is probably the easiest implementation to relate to, so is 543 a good starting point. 544 545 It is extremely simple: 546 547 static DEFINE_RWLOCK(rcu_gp_mutex); 548 549 void rcu_read_lock(void) 550 { 551 read_lock(&rcu_gp_mutex); 552 } 553 554 void rcu_read_unlock(void) 555 { 556 read_unlock(&rcu_gp_mutex); 557 } 558 559 void synchronize_rcu(void) 560 { 561 write_lock(&rcu_gp_mutex); 562 write_unlock(&rcu_gp_mutex); 563 } 564 565 [You can ignore rcu_assign_pointer() and rcu_dereference() without 566 missing much. But here they are anyway. And whatever you do, don't 567 forget about them when submitting patches making use of RCU!] 568 569 #define rcu_assign_pointer(p, v) ({ \ 570 smp_wmb(); \ 571 (p) = (v); \ 572 }) 573 574 #define rcu_dereference(p) ({ \ 575 typeof(p) _________p1 = p; \ 576 smp_read_barrier_depends(); \ 577 (_________p1); \ 578 }) 579 580 581 The rcu_read_lock() and rcu_read_unlock() primitive read-acquire 582 and release a global reader-writer lock. The synchronize_rcu() 583 primitive write-acquires this same lock, then immediately releases 584 it. This means that once synchronize_rcu() exits, all RCU read-side 585 critical sections that were in progress before synchronize_rcu() was 586 called are guaranteed to have completed -- there is no way that 587 synchronize_rcu() would have been able to write-acquire the lock 588 otherwise. 589 590 It is possible to nest rcu_read_lock(), since reader-writer locks may 591 be recursively acquired. Note also that rcu_read_lock() is immune 592 from deadlock (an important property of RCU). The reason for this is 593 that the only thing that can block rcu_read_lock() is a synchronize_rcu(). 594 But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex, 595 so there can be no deadlock cycle. 596 597 Quick Quiz #1: Why is this argument naive? How could a deadlock 598 occur when using this algorithm in a real-world Linux 599 kernel? How could this deadlock be avoided? 600 601 602 5B. "TOY" EXAMPLE #2: CLASSIC RCU 603 604 This section presents a "toy" RCU implementation that is based on 605 "classic RCU". It is also short on performance (but only for updates) and 606 on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT 607 kernels. The definitions of rcu_dereference() and rcu_assign_pointer() 608 are the same as those shown in the preceding section, so they are omitted. 609 610 void rcu_read_lock(void) { } 611 612 void rcu_read_unlock(void) { } 613 614 void synchronize_rcu(void) 615 { 616 int cpu; 617 618 for_each_possible_cpu(cpu) 619 run_on(cpu); 620 } 621 622 Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing. 623 This is the great strength of classic RCU in a non-preemptive kernel: 624 read-side overhead is precisely zero, at least on non-Alpha CPUs. 625 And there is absolutely no way that rcu_read_lock() can possibly 626 participate in a deadlock cycle! 627 628 The implementation of synchronize_rcu() simply schedules itself on each 629 CPU in turn. The run_on() primitive can be implemented straightforwardly 630 in terms of the sched_setaffinity() primitive. Of course, a somewhat less 631 "toy" implementation would restore the affinity upon completion rather 632 than just leaving all tasks running on the last CPU, but when I said 633 "toy", I meant -toy-! 634 635 So how the heck is this supposed to work??? 636 637 Remember that it is illegal to block while in an RCU read-side critical 638 section. Therefore, if a given CPU executes a context switch, we know 639 that it must have completed all preceding RCU read-side critical sections. 640 Once -all- CPUs have executed a context switch, then -all- preceding 641 RCU read-side critical sections will have completed. 642 643 So, suppose that we remove a data item from its structure and then invoke 644 synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed 645 that there are no RCU read-side critical sections holding a reference 646 to that data item, so we can safely reclaim it. 647 648 Quick Quiz #2: Give an example where Classic RCU's read-side 649 overhead is -negative-. 650 651 Quick Quiz #3: If it is illegal to block in an RCU read-side 652 critical section, what the heck do you do in 653 PREEMPT_RT, where normal spinlocks can block??? 654 655 656 6. ANALOGY WITH READER-WRITER LOCKING 657 658 Although RCU can be used in many different ways, a very common use of 659 RCU is analogous to reader-writer locking. The following unified 660 diff shows how closely related RCU and reader-writer locking can be. 661 662 @@ -13,15 +14,15 @@ 663 struct list_head *lp; 664 struct el *p; 665 666 - read_lock(); 667 - list_for_each_entry(p, head, lp) { 668 + rcu_read_lock(); 669 + list_for_each_entry_rcu(p, head, lp) { 670 if (p->key == key) { 671 *result = p->data; 672 - read_unlock(); 673 + rcu_read_unlock(); 674 return 1; 675 } 676 } 677 - read_unlock(); 678 + rcu_read_unlock(); 679 return 0; 680 } 681 682 @@ -29,15 +30,16 @@ 683 { 684 struct el *p; 685 686 - write_lock(&listmutex); 687 + spin_lock(&listmutex); 688 list_for_each_entry(p, head, lp) { 689 if (p->key == key) { 690 - list_del(&p->list); 691 - write_unlock(&listmutex); 692 + list_del_rcu(&p->list); 693 + spin_unlock(&listmutex); 694 + synchronize_rcu(); 695 kfree(p); 696 return 1; 697 } 698 } 699 - write_unlock(&listmutex); 700 + spin_unlock(&listmutex); 701 return 0; 702 } 703 704 Or, for those who prefer a side-by-side listing: 705 706 1 struct el { 1 struct el { 707 2 struct list_head list; 2 struct list_head list; 708 3 long key; 3 long key; 709 4 spinlock_t mutex; 4 spinlock_t mutex; 710 5 int data; 5 int data; 711 6 /* Other data fields */ 6 /* Other data fields */ 712 7 }; 7 }; 713 8 spinlock_t listmutex; 8 spinlock_t listmutex; 714 9 struct el head; 9 struct el head; 715 716 1 int search(long key, int *result) 1 int search(long key, int *result) 717 2 { 2 { 718 3 struct list_head *lp; 3 struct list_head *lp; 719 4 struct el *p; 4 struct el *p; 720 5 5 721 6 read_lock(); 6 rcu_read_lock(); 722 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) { 723 8 if (p->key == key) { 8 if (p->key == key) { 724 9 *result = p->data; 9 *result = p->data; 725 10 read_unlock(); 10 rcu_read_unlock(); 726 11 return 1; 11 return 1; 727 12 } 12 } 728 13 } 13 } 729 14 read_unlock(); 14 rcu_read_unlock(); 730 15 return 0; 15 return 0; 731 16 } 16 } 732 733 1 int delete(long key) 1 int delete(long key) 734 2 { 2 { 735 3 struct el *p; 3 struct el *p; 736 4 4 737 5 write_lock(&listmutex); 5 spin_lock(&listmutex); 738 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) { 739 7 if (p->key == key) { 7 if (p->key == key) { 740 8 list_del(&p->list); 8 list_del_rcu(&p->list); 741 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex); 742 10 synchronize_rcu(); 743 10 kfree(p); 11 kfree(p); 744 11 return 1; 12 return 1; 745 12 } 13 } 746 13 } 14 } 747 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex); 748 15 return 0; 16 return 0; 749 16 } 17 } 750 751 Either way, the differences are quite small. Read-side locking moves 752 to rcu_read_lock() and rcu_read_unlock, update-side locking moves from 753 a reader-writer lock to a simple spinlock, and a synchronize_rcu() 754 precedes the kfree(). 755 756 However, there is one potential catch: the read-side and update-side 757 critical sections can now run concurrently. In many cases, this will 758 not be a problem, but it is necessary to check carefully regardless. 759 For example, if multiple independent list updates must be seen as 760 a single atomic update, converting to RCU will require special care. 761 762 Also, the presence of synchronize_rcu() means that the RCU version of 763 delete() can now block. If this is a problem, there is a callback-based 764 mechanism that never blocks, namely call_rcu(), that can be used in 765 place of synchronize_rcu(). 766 767 768 7. FULL LIST OF RCU APIs 769 770 The RCU APIs are documented in docbook-format header comments in the 771 Linux-kernel source code, but it helps to have a full list of the 772 APIs, since there does not appear to be a way to categorize them 773 in docbook. Here is the list, by category. 774 775 Markers for RCU read-side critical sections: 776 777 rcu_read_lock 778 rcu_read_unlock 779 rcu_read_lock_bh 780 rcu_read_unlock_bh 781 srcu_read_lock 782 srcu_read_unlock 783 784 RCU pointer/list traversal: 785 786 rcu_dereference 787 list_for_each_rcu (to be deprecated in favor of 788 list_for_each_entry_rcu) 789 list_for_each_entry_rcu 790 list_for_each_continue_rcu (to be deprecated in favor of new 791 list_for_each_entry_continue_rcu) 792 hlist_for_each_entry_rcu 793 794 RCU pointer update: 795 796 rcu_assign_pointer 797 list_add_rcu 798 list_add_tail_rcu 799 list_del_rcu 800 list_replace_rcu 801 hlist_del_rcu 802 hlist_add_head_rcu 803 804 RCU grace period: 805 806 synchronize_net 807 synchronize_sched 808 synchronize_rcu 809 synchronize_srcu 810 call_rcu 811 call_rcu_bh 812 813 See the comment headers in the source code (or the docbook generated 814 from them) for more information. 815 816 817 8. ANSWERS TO QUICK QUIZZES 818 819 Quick Quiz #1: Why is this argument naive? How could a deadlock 820 occur when using this algorithm in a real-world Linux 821 kernel? [Referring to the lock-based "toy" RCU 822 algorithm.] 823 824 Answer: Consider the following sequence of events: 825 826 1. CPU 0 acquires some unrelated lock, call it 827 "problematic_lock", disabling irq via 828 spin_lock_irqsave(). 829 830 2. CPU 1 enters synchronize_rcu(), write-acquiring 831 rcu_gp_mutex. 832 833 3. CPU 0 enters rcu_read_lock(), but must wait 834 because CPU 1 holds rcu_gp_mutex. 835 836 4. CPU 1 is interrupted, and the irq handler 837 attempts to acquire problematic_lock. 838 839 The system is now deadlocked. 840 841 One way to avoid this deadlock is to use an approach like 842 that of CONFIG_PREEMPT_RT, where all normal spinlocks 843 become blocking locks, and all irq handlers execute in 844 the context of special tasks. In this case, in step 4 845 above, the irq handler would block, allowing CPU 1 to 846 release rcu_gp_mutex, avoiding the deadlock. 847 848 Even in the absence of deadlock, this RCU implementation 849 allows latency to "bleed" from readers to other 850 readers through synchronize_rcu(). To see this, 851 consider task A in an RCU read-side critical section 852 (thus read-holding rcu_gp_mutex), task B blocked 853 attempting to write-acquire rcu_gp_mutex, and 854 task C blocked in rcu_read_lock() attempting to 855 read_acquire rcu_gp_mutex. Task A's RCU read-side 856 latency is holding up task C, albeit indirectly via 857 task B. 858 859 Realtime RCU implementations therefore use a counter-based 860 approach where tasks in RCU read-side critical sections 861 cannot be blocked by tasks executing synchronize_rcu(). 862 863 Quick Quiz #2: Give an example where Classic RCU's read-side 864 overhead is -negative-. 865 866 Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT 867 kernel where a routing table is used by process-context 868 code, but can be updated by irq-context code (for example, 869 by an "ICMP REDIRECT" packet). The usual way of handling 870 this would be to have the process-context code disable 871 interrupts while searching the routing table. Use of 872 RCU allows such interrupt-disabling to be dispensed with. 873 Thus, without RCU, you pay the cost of disabling interrupts, 874 and with RCU you don't. 875 876 One can argue that the overhead of RCU in this 877 case is negative with respect to the single-CPU 878 interrupt-disabling approach. Others might argue that 879 the overhead of RCU is merely zero, and that replacing 880 the positive overhead of the interrupt-disabling scheme 881 with the zero-overhead RCU scheme does not constitute 882 negative overhead. 883 884 In real life, of course, things are more complex. But 885 even the theoretical possibility of negative overhead for 886 a synchronization primitive is a bit unexpected. ;-) 887 888 Quick Quiz #3: If it is illegal to block in an RCU read-side 889 critical section, what the heck do you do in 890 PREEMPT_RT, where normal spinlocks can block??? 891 892 Answer: Just as PREEMPT_RT permits preemption of spinlock 893 critical sections, it permits preemption of RCU 894 read-side critical sections. It also permits 895 spinlocks blocking while in RCU read-side critical 896 sections. 897 898 Why the apparent inconsistency? Because it is it 899 possible to use priority boosting to keep the RCU 900 grace periods short if need be (for example, if running 901 short of memory). In contrast, if blocking waiting 902 for (say) network reception, there is no way to know 903 what should be boosted. Especially given that the 904 process we need to boost might well be a human being 905 who just went out for a pizza or something. And although 906 a computer-operated cattle prod might arouse serious 907 interest, it might also provoke serious objections. 908 Besides, how does the computer know what pizza parlor 909 the human being went to??? 910 911 912 ACKNOWLEDGEMENTS 913 914 My thanks to the people who helped make this human-readable, including 915 Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern. 916 917 918 For more information, see http://www.rdrop.com/users/paulmck/RCU.