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26
27===========================
28Lockless Ring Buffer Design
29===========================
30
31Copyright 2009 Red Hat Inc.
32
33:Author: Steven Rostedt <srostedt@redhat.com>
34:License: The GNU Free Documentation License, Version 1.2
35 (dual licensed under the GPL v2)
36:Reviewers: Mathieu Desnoyers, Huang Ying, Hidetoshi Seto,
37 and Frederic Weisbecker.
38
39
40Written for: 2.6.31
41
42Terminology used in this Document
43---------------------------------
44
45tail
46 - where new writes happen in the ring buffer.
47
48head
49 - where new reads happen in the ring buffer.
50
51producer
52 - the task that writes into the ring buffer (same as writer)
53
54writer
55 - same as producer
56
57consumer
58 - the task that reads from the buffer (same as reader)
59
60reader
61 - same as consumer.
62
63reader_page
64 - A page outside the ring buffer used solely (for the most part)
65 by the reader.
66
67head_page
68 - a pointer to the page that the reader will use next
69
70tail_page
71 - a pointer to the page that will be written to next
72
73commit_page
74 - a pointer to the page with the last finished non-nested write.
75
76cmpxchg
77 - hardware-assisted atomic transaction that performs the following::
78
79 A = B if previous A == C
80
81 R = cmpxchg(A, C, B) is saying that we replace A with B if and only
82 if current A is equal to C, and we put the old (current)
83 A into R
84
85 R gets the previous A regardless if A is updated with B or not.
86
87 To see if the update was successful a compare of ``R == C``
88 may be used.
89
90The Generic Ring Buffer
91-----------------------
92
93The ring buffer can be used in either an overwrite mode or in
94producer/consumer mode.
95
96Producer/consumer mode is where if the producer were to fill up the
97buffer before the consumer could free up anything, the producer
98will stop writing to the buffer. This will lose most recent events.
99
100Overwrite mode is where if the producer were to fill up the buffer
101before the consumer could free up anything, the producer will
102overwrite the older data. This will lose the oldest events.
103
104No two writers can write at the same time (on the same per-cpu buffer),
105but a writer may interrupt another writer, but it must finish writing
106before the previous writer may continue. This is very important to the
107algorithm. The writers act like a "stack". The way interrupts works
108enforces this behavior::
109
110
111 writer1 start
112 <preempted> writer2 start
113 <preempted> writer3 start
114 writer3 finishes
115 writer2 finishes
116 writer1 finishes
117
118This is very much like a writer being preempted by an interrupt and
119the interrupt doing a write as well.
120
121Readers can happen at any time. But no two readers may run at the
122same time, nor can a reader preempt/interrupt another reader. A reader
123cannot preempt/interrupt a writer, but it may read/consume from the
124buffer at the same time as a writer is writing, but the reader must be
125on another processor to do so. A reader may read on its own processor
126and can be preempted by a writer.
127
128A writer can preempt a reader, but a reader cannot preempt a writer.
129But a reader can read the buffer at the same time (on another processor)
130as a writer.
131
132The ring buffer is made up of a list of pages held together by a linked list.
133
134At initialization a reader page is allocated for the reader that is not
135part of the ring buffer.
136
137The head_page, tail_page and commit_page are all initialized to point
138to the same page.
139
140The reader page is initialized to have its next pointer pointing to
141the head page, and its previous pointer pointing to a page before
142the head page.
143
144The reader has its own page to use. At start up time, this page is
145allocated but is not attached to the list. When the reader wants
146to read from the buffer, if its page is empty (like it is on start-up),
147it will swap its page with the head_page. The old reader page will
148become part of the ring buffer and the head_page will be removed.
149The page after the inserted page (old reader_page) will become the
150new head page.
151
152Once the new page is given to the reader, the reader could do what
153it wants with it, as long as a writer has left that page.
154
155A sample of how the reader page is swapped: Note this does not
156show the head page in the buffer, it is for demonstrating a swap
157only.
158
159::
160
161 +------+
162 |reader| RING BUFFER
163 |page |
164 +------+
165 +---+ +---+ +---+
166 | |-->| |-->| |
167 | |<--| |<--| |
168 +---+ +---+ +---+
169 ^ | ^ |
170 | +-------------+ |
171 +-----------------+
172
173
174 +------+
175 |reader| RING BUFFER
176 |page |-------------------+
177 +------+ v
178 | +---+ +---+ +---+
179 | | |-->| |-->| |
180 | | |<--| |<--| |<-+
181 | +---+ +---+ +---+ |
182 | ^ | ^ | |
183 | | +-------------+ | |
184 | +-----------------+ |
185 +------------------------------------+
186
187 +------+
188 |reader| RING BUFFER
189 |page |-------------------+
190 +------+ <---------------+ v
191 | ^ +---+ +---+ +---+
192 | | | |-->| |-->| |
193 | | | | | |<--| |<-+
194 | | +---+ +---+ +---+ |
195 | | | ^ | |
196 | | +-------------+ | |
197 | +-----------------------------+ |
198 +------------------------------------+
199
200 +------+
201 |buffer| RING BUFFER
202 |page |-------------------+
203 +------+ <---------------+ v
204 | ^ +---+ +---+ +---+
205 | | | | | |-->| |
206 | | New | | | |<--| |<-+
207 | | Reader +---+ +---+ +---+ |
208 | | page ----^ | |
209 | | | |
210 | +-----------------------------+ |
211 +------------------------------------+
212
213
214
215It is possible that the page swapped is the commit page and the tail page,
216if what is in the ring buffer is less than what is held in a buffer page.
217
218::
219
220 reader page commit page tail page
221 | | |
222 v | |
223 +---+ | |
224 | |<----------+ |
225 | |<------------------------+
226 | |------+
227 +---+ |
228 |
229 v
230 +---+ +---+ +---+ +---+
231 <---| |--->| |--->| |--->| |--->
232 --->| |<---| |<---| |<---| |<---
233 +---+ +---+ +---+ +---+
234
235This case is still valid for this algorithm.
236When the writer leaves the page, it simply goes into the ring buffer
237since the reader page still points to the next location in the ring
238buffer.
239
240
241The main pointers:
242
243 reader page
244 - The page used solely by the reader and is not part
245 of the ring buffer (may be swapped in)
246
247 head page
248 - the next page in the ring buffer that will be swapped
249 with the reader page.
250
251 tail page
252 - the page where the next write will take place.
253
254 commit page
255 - the page that last finished a write.
256
257The commit page only is updated by the outermost writer in the
258writer stack. A writer that preempts another writer will not move the
259commit page.
260
261When data is written into the ring buffer, a position is reserved
262in the ring buffer and passed back to the writer. When the writer
263is finished writing data into that position, it commits the write.
264
265Another write (or a read) may take place at anytime during this
266transaction. If another write happens it must finish before continuing
267with the previous write.
268
269
270 Write reserve::
271
272 Buffer page
273 +---------+
274 |written |
275 +---------+ <--- given back to writer (current commit)
276 |reserved |
277 +---------+ <--- tail pointer
278 | empty |
279 +---------+
280
281 Write commit::
282
283 Buffer page
284 +---------+
285 |written |
286 +---------+
287 |written |
288 +---------+ <--- next position for write (current commit)
289 | empty |
290 +---------+
291
292
293 If a write happens after the first reserve::
294
295 Buffer page
296 +---------+
297 |written |
298 +---------+ <-- current commit
299 |reserved |
300 +---------+ <--- given back to second writer
301 |reserved |
302 +---------+ <--- tail pointer
303
304 After second writer commits::
305
306
307 Buffer page
308 +---------+
309 |written |
310 +---------+ <--(last full commit)
311 |reserved |
312 +---------+
313 |pending |
314 |commit |
315 +---------+ <--- tail pointer
316
317 When the first writer commits::
318
319 Buffer page
320 +---------+
321 |written |
322 +---------+
323 |written |
324 +---------+
325 |written |
326 +---------+ <--(last full commit and tail pointer)
327
328
329The commit pointer points to the last write location that was
330committed without preempting another write. When a write that
331preempted another write is committed, it only becomes a pending commit
332and will not be a full commit until all writes have been committed.
333
334The commit page points to the page that has the last full commit.
335The tail page points to the page with the last write (before
336committing).
337
338The tail page is always equal to or after the commit page. It may
339be several pages ahead. If the tail page catches up to the commit
340page then no more writes may take place (regardless of the mode
341of the ring buffer: overwrite and produce/consumer).
342
343The order of pages is::
344
345 head page
346 commit page
347 tail page
348
349Possible scenario::
350
351 tail page
352 head page commit page |
353 | | |
354 v v v
355 +---+ +---+ +---+ +---+
356 <---| |--->| |--->| |--->| |--->
357 --->| |<---| |<---| |<---| |<---
358 +---+ +---+ +---+ +---+
359
360There is a special case that the head page is after either the commit page
361and possibly the tail page. That is when the commit (and tail) page has been
362swapped with the reader page. This is because the head page is always
363part of the ring buffer, but the reader page is not. Whenever there
364has been less than a full page that has been committed inside the ring buffer,
365and a reader swaps out a page, it will be swapping out the commit page.
366
367::
368
369 reader page commit page tail page
370 | | |
371 v | |
372 +---+ | |
373 | |<----------+ |
374 | |<------------------------+
375 | |------+
376 +---+ |
377 |
378 v
379 +---+ +---+ +---+ +---+
380 <---| |--->| |--->| |--->| |--->
381 --->| |<---| |<---| |<---| |<---
382 +---+ +---+ +---+ +---+
383 ^
384 |
385 head page
386
387
388In this case, the head page will not move when the tail and commit
389move back into the ring buffer.
390
391The reader cannot swap a page into the ring buffer if the commit page
392is still on that page. If the read meets the last commit (real commit
393not pending or reserved), then there is nothing more to read.
394The buffer is considered empty until another full commit finishes.
395
396When the tail meets the head page, if the buffer is in overwrite mode,
397the head page will be pushed ahead one. If the buffer is in producer/consumer
398mode, the write will fail.
399
400Overwrite mode::
401
402 tail page
403 |
404 v
405 +---+ +---+ +---+ +---+
406 <---| |--->| |--->| |--->| |--->
407 --->| |<---| |<---| |<---| |<---
408 +---+ +---+ +---+ +---+
409 ^
410 |
411 head page
412
413
414 tail page
415 |
416 v
417 +---+ +---+ +---+ +---+
418 <---| |--->| |--->| |--->| |--->
419 --->| |<---| |<---| |<---| |<---
420 +---+ +---+ +---+ +---+
421 ^
422 |
423 head page
424
425
426 tail page
427 |
428 v
429 +---+ +---+ +---+ +---+
430 <---| |--->| |--->| |--->| |--->
431 --->| |<---| |<---| |<---| |<---
432 +---+ +---+ +---+ +---+
433 ^
434 |
435 head page
436
437Note, the reader page will still point to the previous head page.
438But when a swap takes place, it will use the most recent head page.
439
440
441Making the Ring Buffer Lockless:
442--------------------------------
443
444The main idea behind the lockless algorithm is to combine the moving
445of the head_page pointer with the swapping of pages with the reader.
446State flags are placed inside the pointer to the page. To do this,
447each page must be aligned in memory by 4 bytes. This will allow the 2
448least significant bits of the address to be used as flags, since
449they will always be zero for the address. To get the address,
450simply mask out the flags::
451
452 MASK = ~3
453
454 address & MASK
455
456Two flags will be kept by these two bits:
457
458 HEADER
459 - the page being pointed to is a head page
460
461 UPDATE
462 - the page being pointed to is being updated by a writer
463 and was or is about to be a head page.
464
465::
466
467 reader page
468 |
469 v
470 +---+
471 | |------+
472 +---+ |
473 |
474 v
475 +---+ +---+ +---+ +---+
476 <---| |--->| |-H->| |--->| |--->
477 --->| |<---| |<---| |<---| |<---
478 +---+ +---+ +---+ +---+
479
480
481The above pointer "-H->" would have the HEADER flag set. That is
482the next page is the next page to be swapped out by the reader.
483This pointer means the next page is the head page.
484
485When the tail page meets the head pointer, it will use cmpxchg to
486change the pointer to the UPDATE state::
487
488
489 tail page
490 |
491 v
492 +---+ +---+ +---+ +---+
493 <---| |--->| |-H->| |--->| |--->
494 --->| |<---| |<---| |<---| |<---
495 +---+ +---+ +---+ +---+
496
497 tail page
498 |
499 v
500 +---+ +---+ +---+ +---+
501 <---| |--->| |-U->| |--->| |--->
502 --->| |<---| |<---| |<---| |<---
503 +---+ +---+ +---+ +---+
504
505"-U->" represents a pointer in the UPDATE state.
506
507Any access to the reader will need to take some sort of lock to serialize
508the readers. But the writers will never take a lock to write to the
509ring buffer. This means we only need to worry about a single reader,
510and writes only preempt in "stack" formation.
511
512When the reader tries to swap the page with the ring buffer, it
513will also use cmpxchg. If the flag bit in the pointer to the
514head page does not have the HEADER flag set, the compare will fail
515and the reader will need to look for the new head page and try again.
516Note, the flags UPDATE and HEADER are never set at the same time.
517
518The reader swaps the reader page as follows::
519
520 +------+
521 |reader| RING BUFFER
522 |page |
523 +------+
524 +---+ +---+ +---+
525 | |--->| |--->| |
526 | |<---| |<---| |
527 +---+ +---+ +---+
528 ^ | ^ |
529 | +---------------+ |
530 +-----H-------------+
531
532The reader sets the reader page next pointer as HEADER to the page after
533the head page::
534
535
536 +------+
537 |reader| RING BUFFER
538 |page |-------H-----------+
539 +------+ v
540 | +---+ +---+ +---+
541 | | |--->| |--->| |
542 | | |<---| |<---| |<-+
543 | +---+ +---+ +---+ |
544 | ^ | ^ | |
545 | | +---------------+ | |
546 | +-----H-------------+ |
547 +--------------------------------------+
548
549It does a cmpxchg with the pointer to the previous head page to make it
550point to the reader page. Note that the new pointer does not have the HEADER
551flag set. This action atomically moves the head page forward::
552
553 +------+
554 |reader| RING BUFFER
555 |page |-------H-----------+
556 +------+ v
557 | ^ +---+ +---+ +---+
558 | | | |-->| |-->| |
559 | | | |<--| |<--| |<-+
560 | | +---+ +---+ +---+ |
561 | | | ^ | |
562 | | +-------------+ | |
563 | +-----------------------------+ |
564 +------------------------------------+
565
566After the new head page is set, the previous pointer of the head page is
567updated to the reader page::
568
569 +------+
570 |reader| RING BUFFER
571 |page |-------H-----------+
572 +------+ <---------------+ v
573 | ^ +---+ +---+ +---+
574 | | | |-->| |-->| |
575 | | | | | |<--| |<-+
576 | | +---+ +---+ +---+ |
577 | | | ^ | |
578 | | +-------------+ | |
579 | +-----------------------------+ |
580 +------------------------------------+
581
582 +------+
583 |buffer| RING BUFFER
584 |page |-------H-----------+ <--- New head page
585 +------+ <---------------+ v
586 | ^ +---+ +---+ +---+
587 | | | | | |-->| |
588 | | New | | | |<--| |<-+
589 | | Reader +---+ +---+ +---+ |
590 | | page ----^ | |
591 | | | |
592 | +-----------------------------+ |
593 +------------------------------------+
594
595Another important point: The page that the reader page points back to
596by its previous pointer (the one that now points to the new head page)
597never points back to the reader page. That is because the reader page is
598not part of the ring buffer. Traversing the ring buffer via the next pointers
599will always stay in the ring buffer. Traversing the ring buffer via the
600prev pointers may not.
601
602Note, the way to determine a reader page is simply by examining the previous
603pointer of the page. If the next pointer of the previous page does not
604point back to the original page, then the original page is a reader page::
605
606
607 +--------+
608 | reader | next +----+
609 | page |-------->| |<====== (buffer page)
610 +--------+ +----+
611 | | ^
612 | v | next
613 prev | +----+
614 +------------->| |
615 +----+
616
617The way the head page moves forward:
618
619When the tail page meets the head page and the buffer is in overwrite mode
620and more writes take place, the head page must be moved forward before the
621writer may move the tail page. The way this is done is that the writer
622performs a cmpxchg to convert the pointer to the head page from the HEADER
623flag to have the UPDATE flag set. Once this is done, the reader will
624not be able to swap the head page from the buffer, nor will it be able to
625move the head page, until the writer is finished with the move.
626
627This eliminates any races that the reader can have on the writer. The reader
628must spin, and this is why the reader cannot preempt the writer::
629
630 tail page
631 |
632 v
633 +---+ +---+ +---+ +---+
634 <---| |--->| |-H->| |--->| |--->
635 --->| |<---| |<---| |<---| |<---
636 +---+ +---+ +---+ +---+
637
638 tail page
639 |
640 v
641 +---+ +---+ +---+ +---+
642 <---| |--->| |-U->| |--->| |--->
643 --->| |<---| |<---| |<---| |<---
644 +---+ +---+ +---+ +---+
645
646The following page will be made into the new head page::
647
648 tail page
649 |
650 v
651 +---+ +---+ +---+ +---+
652 <---| |--->| |-U->| |-H->| |--->
653 --->| |<---| |<---| |<---| |<---
654 +---+ +---+ +---+ +---+
655
656After the new head page has been set, we can set the old head page
657pointer back to NORMAL::
658
659 tail page
660 |
661 v
662 +---+ +---+ +---+ +---+
663 <---| |--->| |--->| |-H->| |--->
664 --->| |<---| |<---| |<---| |<---
665 +---+ +---+ +---+ +---+
666
667After the head page has been moved, the tail page may now move forward::
668
669 tail page
670 |
671 v
672 +---+ +---+ +---+ +---+
673 <---| |--->| |--->| |-H->| |--->
674 --->| |<---| |<---| |<---| |<---
675 +---+ +---+ +---+ +---+
676
677
678The above are the trivial updates. Now for the more complex scenarios.
679
680
681As stated before, if enough writes preempt the first write, the
682tail page may make it all the way around the buffer and meet the commit
683page. At this time, we must start dropping writes (usually with some kind
684of warning to the user). But what happens if the commit was still on the
685reader page? The commit page is not part of the ring buffer. The tail page
686must account for this::
687
688
689 reader page commit page
690 | |
691 v |
692 +---+ |
693 | |<----------+
694 | |
695 | |------+
696 +---+ |
697 |
698 v
699 +---+ +---+ +---+ +---+
700 <---| |--->| |-H->| |--->| |--->
701 --->| |<---| |<---| |<---| |<---
702 +---+ +---+ +---+ +---+
703 ^
704 |
705 tail page
706
707If the tail page were to simply push the head page forward, the commit when
708leaving the reader page would not be pointing to the correct page.
709
710The solution to this is to test if the commit page is on the reader page
711before pushing the head page. If it is, then it can be assumed that the
712tail page wrapped the buffer, and we must drop new writes.
713
714This is not a race condition, because the commit page can only be moved
715by the outermost writer (the writer that was preempted).
716This means that the commit will not move while a writer is moving the
717tail page. The reader cannot swap the reader page if it is also being
718used as the commit page. The reader can simply check that the commit
719is off the reader page. Once the commit page leaves the reader page
720it will never go back on it unless a reader does another swap with the
721buffer page that is also the commit page.
722
723
724Nested writes
725-------------
726
727In the pushing forward of the tail page we must first push forward
728the head page if the head page is the next page. If the head page
729is not the next page, the tail page is simply updated with a cmpxchg.
730
731Only writers move the tail page. This must be done atomically to protect
732against nested writers::
733
734 temp_page = tail_page
735 next_page = temp_page->next
736 cmpxchg(tail_page, temp_page, next_page)
737
738The above will update the tail page if it is still pointing to the expected
739page. If this fails, a nested write pushed it forward, the current write
740does not need to push it::
741
742
743 temp page
744 |
745 v
746 tail page
747 |
748 v
749 +---+ +---+ +---+ +---+
750 <---| |--->| |--->| |--->| |--->
751 --->| |<---| |<---| |<---| |<---
752 +---+ +---+ +---+ +---+
753
754Nested write comes in and moves the tail page forward::
755
756 tail page (moved by nested writer)
757 temp page |
758 | |
759 v v
760 +---+ +---+ +---+ +---+
761 <---| |--->| |--->| |--->| |--->
762 --->| |<---| |<---| |<---| |<---
763 +---+ +---+ +---+ +---+
764
765The above would fail the cmpxchg, but since the tail page has already
766been moved forward, the writer will just try again to reserve storage
767on the new tail page.
768
769But the moving of the head page is a bit more complex::
770
771 tail page
772 |
773 v
774 +---+ +---+ +---+ +---+
775 <---| |--->| |-H->| |--->| |--->
776 --->| |<---| |<---| |<---| |<---
777 +---+ +---+ +---+ +---+
778
779The write converts the head page pointer to UPDATE::
780
781 tail page
782 |
783 v
784 +---+ +---+ +---+ +---+
785 <---| |--->| |-U->| |--->| |--->
786 --->| |<---| |<---| |<---| |<---
787 +---+ +---+ +---+ +---+
788
789But if a nested writer preempts here, it will see that the next
790page is a head page, but it is also nested. It will detect that
791it is nested and will save that information. The detection is the
792fact that it sees the UPDATE flag instead of a HEADER or NORMAL
793pointer.
794
795The nested writer will set the new head page pointer::
796
797 tail page
798 |
799 v
800 +---+ +---+ +---+ +---+
801 <---| |--->| |-U->| |-H->| |--->
802 --->| |<---| |<---| |<---| |<---
803 +---+ +---+ +---+ +---+
804
805But it will not reset the update back to normal. Only the writer
806that converted a pointer from HEAD to UPDATE will convert it back
807to NORMAL::
808
809 tail page
810 |
811 v
812 +---+ +---+ +---+ +---+
813 <---| |--->| |-U->| |-H->| |--->
814 --->| |<---| |<---| |<---| |<---
815 +---+ +---+ +---+ +---+
816
817After the nested writer finishes, the outermost writer will convert
818the UPDATE pointer to NORMAL::
819
820
821 tail page
822 |
823 v
824 +---+ +---+ +---+ +---+
825 <---| |--->| |--->| |-H->| |--->
826 --->| |<---| |<---| |<---| |<---
827 +---+ +---+ +---+ +---+
828
829
830It can be even more complex if several nested writes came in and moved
831the tail page ahead several pages::
832
833
834 (first writer)
835
836 tail page
837 |
838 v
839 +---+ +---+ +---+ +---+
840 <---| |--->| |-H->| |--->| |--->
841 --->| |<---| |<---| |<---| |<---
842 +---+ +---+ +---+ +---+
843
844The write converts the head page pointer to UPDATE::
845
846 tail page
847 |
848 v
849 +---+ +---+ +---+ +---+
850 <---| |--->| |-U->| |--->| |--->
851 --->| |<---| |<---| |<---| |<---
852 +---+ +---+ +---+ +---+
853
854Next writer comes in, and sees the update and sets up the new
855head page::
856
857 (second writer)
858
859 tail page
860 |
861 v
862 +---+ +---+ +---+ +---+
863 <---| |--->| |-U->| |-H->| |--->
864 --->| |<---| |<---| |<---| |<---
865 +---+ +---+ +---+ +---+
866
867The nested writer moves the tail page forward. But does not set the old
868update page to NORMAL because it is not the outermost writer::
869
870 tail page
871 |
872 v
873 +---+ +---+ +---+ +---+
874 <---| |--->| |-U->| |-H->| |--->
875 --->| |<---| |<---| |<---| |<---
876 +---+ +---+ +---+ +---+
877
878Another writer preempts and sees the page after the tail page is a head page.
879It changes it from HEAD to UPDATE::
880
881 (third writer)
882
883 tail page
884 |
885 v
886 +---+ +---+ +---+ +---+
887 <---| |--->| |-U->| |-U->| |--->
888 --->| |<---| |<---| |<---| |<---
889 +---+ +---+ +---+ +---+
890
891The writer will move the head page forward::
892
893
894 (third writer)
895
896 tail page
897 |
898 v
899 +---+ +---+ +---+ +---+
900 <---| |--->| |-U->| |-U->| |-H->
901 --->| |<---| |<---| |<---| |<---
902 +---+ +---+ +---+ +---+
903
904But now that the third writer did change the HEAD flag to UPDATE it
905will convert it to normal::
906
907
908 (third writer)
909
910 tail page
911 |
912 v
913 +---+ +---+ +---+ +---+
914 <---| |--->| |-U->| |--->| |-H->
915 --->| |<---| |<---| |<---| |<---
916 +---+ +---+ +---+ +---+
917
918
919Then it will move the tail page, and return back to the second writer::
920
921
922 (second writer)
923
924 tail page
925 |
926 v
927 +---+ +---+ +---+ +---+
928 <---| |--->| |-U->| |--->| |-H->
929 --->| |<---| |<---| |<---| |<---
930 +---+ +---+ +---+ +---+
931
932
933The second writer will fail to move the tail page because it was already
934moved, so it will try again and add its data to the new tail page.
935It will return to the first writer::
936
937
938 (first writer)
939
940 tail page
941 |
942 v
943 +---+ +---+ +---+ +---+
944 <---| |--->| |-U->| |--->| |-H->
945 --->| |<---| |<---| |<---| |<---
946 +---+ +---+ +---+ +---+
947
948The first writer cannot know atomically if the tail page moved
949while it updates the HEAD page. It will then update the head page to
950what it thinks is the new head page::
951
952
953 (first writer)
954
955 tail page
956 |
957 v
958 +---+ +---+ +---+ +---+
959 <---| |--->| |-U->| |-H->| |-H->
960 --->| |<---| |<---| |<---| |<---
961 +---+ +---+ +---+ +---+
962
963Since the cmpxchg returns the old value of the pointer the first writer
964will see it succeeded in updating the pointer from NORMAL to HEAD.
965But as we can see, this is not good enough. It must also check to see
966if the tail page is either where it use to be or on the next page::
967
968
969 (first writer)
970
971 A B tail page
972 | | |
973 v v v
974 +---+ +---+ +---+ +---+
975 <---| |--->| |-U->| |-H->| |-H->
976 --->| |<---| |<---| |<---| |<---
977 +---+ +---+ +---+ +---+
978
979If tail page != A and tail page != B, then it must reset the pointer
980back to NORMAL. The fact that it only needs to worry about nested
981writers means that it only needs to check this after setting the HEAD page::
982
983
984 (first writer)
985
986 A B tail page
987 | | |
988 v v v
989 +---+ +---+ +---+ +---+
990 <---| |--->| |-U->| |--->| |-H->
991 --->| |<---| |<---| |<---| |<---
992 +---+ +---+ +---+ +---+
993
994Now the writer can update the head page. This is also why the head page must
995remain in UPDATE and only reset by the outermost writer. This prevents
996the reader from seeing the incorrect head page::
997
998
999 (first writer)
1000
1001 A B tail page
1002 | | |
1003 v v v
1004 +---+ +---+ +---+ +---+
1005 <---| |--->| |--->| |--->| |-H->
1006 --->| |<---| |<---| |<---| |<---
1007 +---+ +---+ +---+ +---+