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1 Lockless Ring Buffer Design
2 ===========================
3
4Copyright 2009 Red Hat Inc.
5 Author: Steven Rostedt <srostedt@redhat.com>
6 License: The GNU Free Documentation License, Version 1.2
7 (dual licensed under the GPL v2)
8Reviewers: Mathieu Desnoyers, Huang Ying, Hidetoshi Seto,
9 and Frederic Weisbecker.
10
11
12Written for: 2.6.31
13
14Terminology used in this Document
15---------------------------------
16
17tail - where new writes happen in the ring buffer.
18
19head - where new reads happen in the ring buffer.
20
21producer - the task that writes into the ring buffer (same as writer)
22
23writer - same as producer
24
25consumer - the task that reads from the buffer (same as reader)
26
27reader - same as consumer.
28
29reader_page - A page outside the ring buffer used solely (for the most part)
30 by the reader.
31
32head_page - a pointer to the page that the reader will use next
33
34tail_page - a pointer to the page that will be written to next
35
36commit_page - a pointer to the page with the last finished non nested write.
37
38cmpxchg - hardware assisted atomic transaction that performs the following:
39
40 A = B iff previous A == C
41
42 R = cmpxchg(A, C, B) is saying that we replace A with B if and only if
43 current A is equal to C, and we put the old (current) A into R
44
45 R gets the previous A regardless if A is updated with B or not.
46
47 To see if the update was successful a compare of R == C may be used.
48
49The Generic Ring Buffer
50-----------------------
51
52The ring buffer can be used in either an overwrite mode or in
53producer/consumer mode.
54
55Producer/consumer mode is where the producer were to fill up the
56buffer before the consumer could free up anything, the producer
57will stop writing to the buffer. This will lose most recent events.
58
59Overwrite mode is where the produce were to fill up the buffer
60before the consumer could free up anything, the producer will
61overwrite the older data. This will lose the oldest events.
62
63No two writers can write at the same time (on the same per cpu buffer),
64but a writer may interrupt another writer, but it must finish writing
65before the previous writer may continue. This is very important to the
66algorithm. The writers act like a "stack". The way interrupts works
67enforces this behavior.
68
69
70 writer1 start
71 <preempted> writer2 start
72 <preempted> writer3 start
73 writer3 finishes
74 writer2 finishes
75 writer1 finishes
76
77This is very much like a writer being preempted by an interrupt and
78the interrupt doing a write as well.
79
80Readers can happen at any time. But no two readers may run at the
81same time, nor can a reader preempt/interrupt another reader. A reader
82can not preempt/interrupt a writer, but it may read/consume from the
83buffer at the same time as a writer is writing, but the reader must be
84on another processor to do so. A reader may read on its own processor
85and can be preempted by a writer.
86
87A writer can preempt a reader, but a reader can not preempt a writer.
88But a reader can read the buffer at the same time (on another processor)
89as a writer.
90
91The ring buffer is made up of a list of pages held together by a link list.
92
93At initialization a reader page is allocated for the reader that is not
94part of the ring buffer.
95
96The head_page, tail_page and commit_page are all initialized to point
97to the same page.
98
99The reader page is initialized to have its next pointer pointing to
100the head page, and its previous pointer pointing to a page before
101the head page.
102
103The reader has its own page to use. At start up time, this page is
104allocated but is not attached to the list. When the reader wants
105to read from the buffer, if its page is empty (like it is on start up)
106it will swap its page with the head_page. The old reader page will
107become part of the ring buffer and the head_page will be removed.
108The page after the inserted page (old reader_page) will become the
109new head page.
110
111Once the new page is given to the reader, the reader could do what
112it wants with it, as long as a writer has left that page.
113
114A sample of how the reader page is swapped: Note this does not
115show the head page in the buffer, it is for demonstrating a swap
116only.
117
118 +------+
119 |reader| RING BUFFER
120 |page |
121 +------+
122 +---+ +---+ +---+
123 | |-->| |-->| |
124 | |<--| |<--| |
125 +---+ +---+ +---+
126 ^ | ^ |
127 | +-------------+ |
128 +-----------------+
129
130
131 +------+
132 |reader| RING BUFFER
133 |page |-------------------+
134 +------+ v
135 | +---+ +---+ +---+
136 | | |-->| |-->| |
137 | | |<--| |<--| |<-+
138 | +---+ +---+ +---+ |
139 | ^ | ^ | |
140 | | +-------------+ | |
141 | +-----------------+ |
142 +------------------------------------+
143
144 +------+
145 |reader| RING BUFFER
146 |page |-------------------+
147 +------+ <---------------+ v
148 | ^ +---+ +---+ +---+
149 | | | |-->| |-->| |
150 | | | | | |<--| |<-+
151 | | +---+ +---+ +---+ |
152 | | | ^ | |
153 | | +-------------+ | |
154 | +-----------------------------+ |
155 +------------------------------------+
156
157 +------+
158 |buffer| RING BUFFER
159 |page |-------------------+
160 +------+ <---------------+ v
161 | ^ +---+ +---+ +---+
162 | | | | | |-->| |
163 | | New | | | |<--| |<-+
164 | | Reader +---+ +---+ +---+ |
165 | | page ----^ | |
166 | | | |
167 | +-----------------------------+ |
168 +------------------------------------+
169
170
171
172It is possible that the page swapped is the commit page and the tail page,
173if what is in the ring buffer is less than what is held in a buffer page.
174
175
176 reader page commit page tail page
177 | | |
178 v | |
179 +---+ | |
180 | |<----------+ |
181 | |<------------------------+
182 | |------+
183 +---+ |
184 |
185 v
186 +---+ +---+ +---+ +---+
187<---| |--->| |--->| |--->| |--->
188--->| |<---| |<---| |<---| |<---
189 +---+ +---+ +---+ +---+
190
191This case is still valid for this algorithm.
192When the writer leaves the page, it simply goes into the ring buffer
193since the reader page still points to the next location in the ring
194buffer.
195
196
197The main pointers:
198
199 reader page - The page used solely by the reader and is not part
200 of the ring buffer (may be swapped in)
201
202 head page - the next page in the ring buffer that will be swapped
203 with the reader page.
204
205 tail page - the page where the next write will take place.
206
207 commit page - the page that last finished a write.
208
209The commit page only is updated by the outer most writer in the
210writer stack. A writer that preempts another writer will not move the
211commit page.
212
213When data is written into the ring buffer, a position is reserved
214in the ring buffer and passed back to the writer. When the writer
215is finished writing data into that position, it commits the write.
216
217Another write (or a read) may take place at anytime during this
218transaction. If another write happens it must finish before continuing
219with the previous write.
220
221
222 Write reserve:
223
224 Buffer page
225 +---------+
226 |written |
227 +---------+ <--- given back to writer (current commit)
228 |reserved |
229 +---------+ <--- tail pointer
230 | empty |
231 +---------+
232
233 Write commit:
234
235 Buffer page
236 +---------+
237 |written |
238 +---------+
239 |written |
240 +---------+ <--- next positon for write (current commit)
241 | empty |
242 +---------+
243
244
245 If a write happens after the first reserve:
246
247 Buffer page
248 +---------+
249 |written |
250 +---------+ <-- current commit
251 |reserved |
252 +---------+ <--- given back to second writer
253 |reserved |
254 +---------+ <--- tail pointer
255
256 After second writer commits:
257
258
259 Buffer page
260 +---------+
261 |written |
262 +---------+ <--(last full commit)
263 |reserved |
264 +---------+
265 |pending |
266 |commit |
267 +---------+ <--- tail pointer
268
269 When the first writer commits:
270
271 Buffer page
272 +---------+
273 |written |
274 +---------+
275 |written |
276 +---------+
277 |written |
278 +---------+ <--(last full commit and tail pointer)
279
280
281The commit pointer points to the last write location that was
282committed without preempting another write. When a write that
283preempted another write is committed, it only becomes a pending commit
284and will not be a full commit till all writes have been committed.
285
286The commit page points to the page that has the last full commit.
287The tail page points to the page with the last write (before
288committing).
289
290The tail page is always equal to or after the commit page. It may
291be several pages ahead. If the tail page catches up to the commit
292page then no more writes may take place (regardless of the mode
293of the ring buffer: overwrite and produce/consumer).
294
295The order of pages are:
296
297 head page
298 commit page
299 tail page
300
301Possible scenario:
302 tail page
303 head page commit page |
304 | | |
305 v v v
306 +---+ +---+ +---+ +---+
307<---| |--->| |--->| |--->| |--->
308--->| |<---| |<---| |<---| |<---
309 +---+ +---+ +---+ +---+
310
311There is a special case that the head page is after either the commit page
312and possibly the tail page. That is when the commit (and tail) page has been
313swapped with the reader page. This is because the head page is always
314part of the ring buffer, but the reader page is not. When ever there
315has been less than a full page that has been committed inside the ring buffer,
316and a reader swaps out a page, it will be swapping out the commit page.
317
318
319 reader page commit page tail page
320 | | |
321 v | |
322 +---+ | |
323 | |<----------+ |
324 | |<------------------------+
325 | |------+
326 +---+ |
327 |
328 v
329 +---+ +---+ +---+ +---+
330<---| |--->| |--->| |--->| |--->
331--->| |<---| |<---| |<---| |<---
332 +---+ +---+ +---+ +---+
333 ^
334 |
335 head page
336
337
338In this case, the head page will not move when the tail and commit
339move back into the ring buffer.
340
341The reader can not swap a page into the ring buffer if the commit page
342is still on that page. If the read meets the last commit (real commit
343not pending or reserved), then there is nothing more to read.
344The buffer is considered empty until another full commit finishes.
345
346When the tail meets the head page, if the buffer is in overwrite mode,
347the head page will be pushed ahead one. If the buffer is in producer/consumer
348mode, the write will fail.
349
350Overwrite mode:
351
352 tail page
353 |
354 v
355 +---+ +---+ +---+ +---+
356<---| |--->| |--->| |--->| |--->
357--->| |<---| |<---| |<---| |<---
358 +---+ +---+ +---+ +---+
359 ^
360 |
361 head page
362
363
364 tail page
365 |
366 v
367 +---+ +---+ +---+ +---+
368<---| |--->| |--->| |--->| |--->
369--->| |<---| |<---| |<---| |<---
370 +---+ +---+ +---+ +---+
371 ^
372 |
373 head page
374
375
376 tail page
377 |
378 v
379 +---+ +---+ +---+ +---+
380<---| |--->| |--->| |--->| |--->
381--->| |<---| |<---| |<---| |<---
382 +---+ +---+ +---+ +---+
383 ^
384 |
385 head page
386
387Note, the reader page will still point to the previous head page.
388But when a swap takes place, it will use the most recent head page.
389
390
391Making the Ring Buffer Lockless:
392--------------------------------
393
394The main idea behind the lockless algorithm is to combine the moving
395of the head_page pointer with the swapping of pages with the reader.
396State flags are placed inside the pointer to the page. To do this,
397each page must be aligned in memory by 4 bytes. This will allow the 2
398least significant bits of the address to be used as flags. Since
399they will always be zero for the address. To get the address,
400simply mask out the flags.
401
402 MASK = ~3
403
404 address & MASK
405
406Two flags will be kept by these two bits:
407
408 HEADER - the page being pointed to is a head page
409
410 UPDATE - the page being pointed to is being updated by a writer
411 and was or is about to be a head page.
412
413
414 reader page
415 |
416 v
417 +---+
418 | |------+
419 +---+ |
420 |
421 v
422 +---+ +---+ +---+ +---+
423<---| |--->| |-H->| |--->| |--->
424--->| |<---| |<---| |<---| |<---
425 +---+ +---+ +---+ +---+
426
427
428The above pointer "-H->" would have the HEADER flag set. That is
429the next page is the next page to be swapped out by the reader.
430This pointer means the next page is the head page.
431
432When the tail page meets the head pointer, it will use cmpxchg to
433change the pointer to the UPDATE state:
434
435
436 tail page
437 |
438 v
439 +---+ +---+ +---+ +---+
440<---| |--->| |-H->| |--->| |--->
441--->| |<---| |<---| |<---| |<---
442 +---+ +---+ +---+ +---+
443
444 tail page
445 |
446 v
447 +---+ +---+ +---+ +---+
448<---| |--->| |-U->| |--->| |--->
449--->| |<---| |<---| |<---| |<---
450 +---+ +---+ +---+ +---+
451
452"-U->" represents a pointer in the UPDATE state.
453
454Any access to the reader will need to take some sort of lock to serialize
455the readers. But the writers will never take a lock to write to the
456ring buffer. This means we only need to worry about a single reader,
457and writes only preempt in "stack" formation.
458
459When the reader tries to swap the page with the ring buffer, it
460will also use cmpxchg. If the flag bit in the pointer to the
461head page does not have the HEADER flag set, the compare will fail
462and the reader will need to look for the new head page and try again.
463Note, the flag UPDATE and HEADER are never set at the same time.
464
465The reader swaps the reader page as follows:
466
467 +------+
468 |reader| RING BUFFER
469 |page |
470 +------+
471 +---+ +---+ +---+
472 | |--->| |--->| |
473 | |<---| |<---| |
474 +---+ +---+ +---+
475 ^ | ^ |
476 | +---------------+ |
477 +-----H-------------+
478
479The reader sets the reader page next pointer as HEADER to the page after
480the head page.
481
482
483 +------+
484 |reader| RING BUFFER
485 |page |-------H-----------+
486 +------+ v
487 | +---+ +---+ +---+
488 | | |--->| |--->| |
489 | | |<---| |<---| |<-+
490 | +---+ +---+ +---+ |
491 | ^ | ^ | |
492 | | +---------------+ | |
493 | +-----H-------------+ |
494 +--------------------------------------+
495
496It does a cmpxchg with the pointer to the previous head page to make it
497point to the reader page. Note that the new pointer does not have the HEADER
498flag set. This action atomically moves the head page forward.
499
500 +------+
501 |reader| RING BUFFER
502 |page |-------H-----------+
503 +------+ v
504 | ^ +---+ +---+ +---+
505 | | | |-->| |-->| |
506 | | | |<--| |<--| |<-+
507 | | +---+ +---+ +---+ |
508 | | | ^ | |
509 | | +-------------+ | |
510 | +-----------------------------+ |
511 +------------------------------------+
512
513After the new head page is set, the previous pointer of the head page is
514updated to the reader page.
515
516 +------+
517 |reader| RING BUFFER
518 |page |-------H-----------+
519 +------+ <---------------+ v
520 | ^ +---+ +---+ +---+
521 | | | |-->| |-->| |
522 | | | | | |<--| |<-+
523 | | +---+ +---+ +---+ |
524 | | | ^ | |
525 | | +-------------+ | |
526 | +-----------------------------+ |
527 +------------------------------------+
528
529 +------+
530 |buffer| RING BUFFER
531 |page |-------H-----------+ <--- New head page
532 +------+ <---------------+ v
533 | ^ +---+ +---+ +---+
534 | | | | | |-->| |
535 | | New | | | |<--| |<-+
536 | | Reader +---+ +---+ +---+ |
537 | | page ----^ | |
538 | | | |
539 | +-----------------------------+ |
540 +------------------------------------+
541
542Another important point. The page that the reader page points back to
543by its previous pointer (the one that now points to the new head page)
544never points back to the reader page. That is because the reader page is
545not part of the ring buffer. Traversing the ring buffer via the next pointers
546will always stay in the ring buffer. Traversing the ring buffer via the
547prev pointers may not.
548
549Note, the way to determine a reader page is simply by examining the previous
550pointer of the page. If the next pointer of the previous page does not
551point back to the original page, then the original page is a reader page:
552
553
554 +--------+
555 | reader | next +----+
556 | page |-------->| |<====== (buffer page)
557 +--------+ +----+
558 | | ^
559 | v | next
560 prev | +----+
561 +------------->| |
562 +----+
563
564The way the head page moves forward:
565
566When the tail page meets the head page and the buffer is in overwrite mode
567and more writes take place, the head page must be moved forward before the
568writer may move the tail page. The way this is done is that the writer
569performs a cmpxchg to convert the pointer to the head page from the HEADER
570flag to have the UPDATE flag set. Once this is done, the reader will
571not be able to swap the head page from the buffer, nor will it be able to
572move the head page, until the writer is finished with the move.
573
574This eliminates any races that the reader can have on the writer. The reader
575must spin, and this is why the reader can not preempt the writer.
576
577 tail page
578 |
579 v
580 +---+ +---+ +---+ +---+
581<---| |--->| |-H->| |--->| |--->
582--->| |<---| |<---| |<---| |<---
583 +---+ +---+ +---+ +---+
584
585 tail page
586 |
587 v
588 +---+ +---+ +---+ +---+
589<---| |--->| |-U->| |--->| |--->
590--->| |<---| |<---| |<---| |<---
591 +---+ +---+ +---+ +---+
592
593The following page will be made into the new head page.
594
595 tail page
596 |
597 v
598 +---+ +---+ +---+ +---+
599<---| |--->| |-U->| |-H->| |--->
600--->| |<---| |<---| |<---| |<---
601 +---+ +---+ +---+ +---+
602
603After the new head page has been set, we can set the old head page
604pointer back to NORMAL.
605
606 tail page
607 |
608 v
609 +---+ +---+ +---+ +---+
610<---| |--->| |--->| |-H->| |--->
611--->| |<---| |<---| |<---| |<---
612 +---+ +---+ +---+ +---+
613
614After the head page has been moved, the tail page may now move forward.
615
616 tail page
617 |
618 v
619 +---+ +---+ +---+ +---+
620<---| |--->| |--->| |-H->| |--->
621--->| |<---| |<---| |<---| |<---
622 +---+ +---+ +---+ +---+
623
624
625The above are the trivial updates. Now for the more complex scenarios.
626
627
628As stated before, if enough writes preempt the first write, the
629tail page may make it all the way around the buffer and meet the commit
630page. At this time, we must start dropping writes (usually with some kind
631of warning to the user). But what happens if the commit was still on the
632reader page? The commit page is not part of the ring buffer. The tail page
633must account for this.
634
635
636 reader page commit page
637 | |
638 v |
639 +---+ |
640 | |<----------+
641 | |
642 | |------+
643 +---+ |
644 |
645 v
646 +---+ +---+ +---+ +---+
647<---| |--->| |-H->| |--->| |--->
648--->| |<---| |<---| |<---| |<---
649 +---+ +---+ +---+ +---+
650 ^
651 |
652 tail page
653
654If the tail page were to simply push the head page forward, the commit when
655leaving the reader page would not be pointing to the correct page.
656
657The solution to this is to test if the commit page is on the reader page
658before pushing the head page. If it is, then it can be assumed that the
659tail page wrapped the buffer, and we must drop new writes.
660
661This is not a race condition, because the commit page can only be moved
662by the outter most writer (the writer that was preempted).
663This means that the commit will not move while a writer is moving the
664tail page. The reader can not swap the reader page if it is also being
665used as the commit page. The reader can simply check that the commit
666is off the reader page. Once the commit page leaves the reader page
667it will never go back on it unless a reader does another swap with the
668buffer page that is also the commit page.
669
670
671Nested writes
672-------------
673
674In the pushing forward of the tail page we must first push forward
675the head page if the head page is the next page. If the head page
676is not the next page, the tail page is simply updated with a cmpxchg.
677
678Only writers move the tail page. This must be done atomically to protect
679against nested writers.
680
681 temp_page = tail_page
682 next_page = temp_page->next
683 cmpxchg(tail_page, temp_page, next_page)
684
685The above will update the tail page if it is still pointing to the expected
686page. If this fails, a nested write pushed it forward, the the current write
687does not need to push it.
688
689
690 temp page
691 |
692 v
693 tail page
694 |
695 v
696 +---+ +---+ +---+ +---+
697<---| |--->| |--->| |--->| |--->
698--->| |<---| |<---| |<---| |<---
699 +---+ +---+ +---+ +---+
700
701Nested write comes in and moves the tail page forward:
702
703 tail page (moved by nested writer)
704 temp page |
705 | |
706 v v
707 +---+ +---+ +---+ +---+
708<---| |--->| |--->| |--->| |--->
709--->| |<---| |<---| |<---| |<---
710 +---+ +---+ +---+ +---+
711
712The above would fail the cmpxchg, but since the tail page has already
713been moved forward, the writer will just try again to reserve storage
714on the new tail page.
715
716But the moving of the head page is a bit more complex.
717
718 tail page
719 |
720 v
721 +---+ +---+ +---+ +---+
722<---| |--->| |-H->| |--->| |--->
723--->| |<---| |<---| |<---| |<---
724 +---+ +---+ +---+ +---+
725
726The write converts the head page pointer to UPDATE.
727
728 tail page
729 |
730 v
731 +---+ +---+ +---+ +---+
732<---| |--->| |-U->| |--->| |--->
733--->| |<---| |<---| |<---| |<---
734 +---+ +---+ +---+ +---+
735
736But if a nested writer preempts here. It will see that the next
737page is a head page, but it is also nested. It will detect that
738it is nested and will save that information. The detection is the
739fact that it sees the UPDATE flag instead of a HEADER or NORMAL
740pointer.
741
742The nested writer will set the new head page pointer.
743
744 tail page
745 |
746 v
747 +---+ +---+ +---+ +---+
748<---| |--->| |-U->| |-H->| |--->
749--->| |<---| |<---| |<---| |<---
750 +---+ +---+ +---+ +---+
751
752But it will not reset the update back to normal. Only the writer
753that converted a pointer from HEAD to UPDATE will convert it back
754to NORMAL.
755
756 tail page
757 |
758 v
759 +---+ +---+ +---+ +---+
760<---| |--->| |-U->| |-H->| |--->
761--->| |<---| |<---| |<---| |<---
762 +---+ +---+ +---+ +---+
763
764After the nested writer finishes, the outer most writer will convert
765the UPDATE pointer to NORMAL.
766
767
768 tail page
769 |
770 v
771 +---+ +---+ +---+ +---+
772<---| |--->| |--->| |-H->| |--->
773--->| |<---| |<---| |<---| |<---
774 +---+ +---+ +---+ +---+
775
776
777It can be even more complex if several nested writes came in and moved
778the tail page ahead several pages:
779
780
781(first writer)
782
783 tail page
784 |
785 v
786 +---+ +---+ +---+ +---+
787<---| |--->| |-H->| |--->| |--->
788--->| |<---| |<---| |<---| |<---
789 +---+ +---+ +---+ +---+
790
791The write converts the head page pointer to UPDATE.
792
793 tail page
794 |
795 v
796 +---+ +---+ +---+ +---+
797<---| |--->| |-U->| |--->| |--->
798--->| |<---| |<---| |<---| |<---
799 +---+ +---+ +---+ +---+
800
801Next writer comes in, and sees the update and sets up the new
802head page.
803
804(second writer)
805
806 tail page
807 |
808 v
809 +---+ +---+ +---+ +---+
810<---| |--->| |-U->| |-H->| |--->
811--->| |<---| |<---| |<---| |<---
812 +---+ +---+ +---+ +---+
813
814The nested writer moves the tail page forward. But does not set the old
815update page to NORMAL because it is not the outer most writer.
816
817 tail page
818 |
819 v
820 +---+ +---+ +---+ +---+
821<---| |--->| |-U->| |-H->| |--->
822--->| |<---| |<---| |<---| |<---
823 +---+ +---+ +---+ +---+
824
825Another writer preempts and sees the page after the tail page is a head page.
826It changes it from HEAD to UPDATE.
827
828(third writer)
829
830 tail page
831 |
832 v
833 +---+ +---+ +---+ +---+
834<---| |--->| |-U->| |-U->| |--->
835--->| |<---| |<---| |<---| |<---
836 +---+ +---+ +---+ +---+
837
838The writer will move the head page forward:
839
840
841(third writer)
842
843 tail page
844 |
845 v
846 +---+ +---+ +---+ +---+
847<---| |--->| |-U->| |-U->| |-H->
848--->| |<---| |<---| |<---| |<---
849 +---+ +---+ +---+ +---+
850
851But now that the third writer did change the HEAD flag to UPDATE it
852will convert it to normal:
853
854
855(third writer)
856
857 tail page
858 |
859 v
860 +---+ +---+ +---+ +---+
861<---| |--->| |-U->| |--->| |-H->
862--->| |<---| |<---| |<---| |<---
863 +---+ +---+ +---+ +---+
864
865
866Then it will move the tail page, and return back to the second writer.
867
868
869(second writer)
870
871 tail page
872 |
873 v
874 +---+ +---+ +---+ +---+
875<---| |--->| |-U->| |--->| |-H->
876--->| |<---| |<---| |<---| |<---
877 +---+ +---+ +---+ +---+
878
879
880The second writer will fail to move the tail page because it was already
881moved, so it will try again and add its data to the new tail page.
882It will return to the first writer.
883
884
885(first writer)
886
887 tail page
888 |
889 v
890 +---+ +---+ +---+ +---+
891<---| |--->| |-U->| |--->| |-H->
892--->| |<---| |<---| |<---| |<---
893 +---+ +---+ +---+ +---+
894
895The first writer can not know atomically test if the tail page moved
896while it updates the HEAD page. It will then update the head page to
897what it thinks is the new head page.
898
899
900(first writer)
901
902 tail page
903 |
904 v
905 +---+ +---+ +---+ +---+
906<---| |--->| |-U->| |-H->| |-H->
907--->| |<---| |<---| |<---| |<---
908 +---+ +---+ +---+ +---+
909
910Since the cmpxchg returns the old value of the pointer the first writer
911will see it succeeded in updating the pointer from NORMAL to HEAD.
912But as we can see, this is not good enough. It must also check to see
913if the tail page is either where it use to be or on the next page:
914
915
916(first writer)
917
918 A B tail page
919 | | |
920 v v v
921 +---+ +---+ +---+ +---+
922<---| |--->| |-U->| |-H->| |-H->
923--->| |<---| |<---| |<---| |<---
924 +---+ +---+ +---+ +---+
925
926If tail page != A and tail page does not equal B, then it must reset the
927pointer back to NORMAL. The fact that it only needs to worry about
928nested writers, it only needs to check this after setting the HEAD page.
929
930
931(first writer)
932
933 A B tail page
934 | | |
935 v v v
936 +---+ +---+ +---+ +---+
937<---| |--->| |-U->| |--->| |-H->
938--->| |<---| |<---| |<---| |<---
939 +---+ +---+ +---+ +---+
940
941Now the writer can update the head page. This is also why the head page must
942remain in UPDATE and only reset by the outer most writer. This prevents
943the reader from seeing the incorrect head page.
944
945
946(first writer)
947
948 A B tail page
949 | | |
950 v v v
951 +---+ +---+ +---+ +---+
952<---| |--->| |--->| |--->| |-H->
953--->| |<---| |<---| |<---| |<---
954 +---+ +---+ +---+ +---+
955
generated by cgit v1.2.2 (git 2.25.0) at 2024-12-11 04:51:03 -0500