https://www.kernel.org/doc/Documentation/trace/ring-buffer-design.txt          Lockless Ring Buffer Design        ===========================Copyright 2009 Red Hat Inc.   Author:   Steven Rostedt <srostedt@redhat.com>  License:   The GNU Free Documentation License, Version 1.2               (dual licensed under the GPL v2)Reviewers:   Mathieu Desnoyers, Huang Ying, Hidetoshi Seto,         and Frederic Weisbecker.Written for: 2.6.31Terminology used in this Document---------------------------------tail - where new writes happen in the ring buffer.head - where new reads happen in the ring buffer.producer - the task that writes into the ring buffer (same as writer)writer - same as producerconsumer - the task that reads from the buffer (same as reader)reader - same as consumer.reader_page - A page outside the ring buffer used solely (for the most part) by the reader.head_page - a pointer to the page that the reader will use nexttail_page - a pointer to the page that will be written to nextcommit_page - a pointer to the page with the last finished non-nested write.cmpxchg - hardware-assisted atomic transaction that performs the following:   A = B iff previous A == C   R = cmpxchg(A, C, B) is saying that we replace A with B    if and only if current A is equal to C, and we put the old (current) A into R   R gets the previous A regardless if A is updated with B or not.   To see if the update was successful a compare of R == C may be used.The Generic Ring Buffer-----------------------The ring buffer can be used in either an overwrite mode or inproducer/consumer mode.Producer/consumer mode is where if the producer were to fill up thebuffer before the consumer could free up anything, the producerwill stop writing to the buffer. This will lose most recent events.Overwrite mode is where if the producer were to fill up the bufferbefore the consumer could free up anything, the producer willoverwrite the older data. This will lose the oldest events.No two writers can write at the same time (on the same per-cpu buffer),but a writer may interrupt another writer, but it must finish writingbefore the previous writer may continue. This is very important to thealgorithm. The writers act like a "stack". The way interrupts worksenforces this behavior.  writer1 start     <preempted> writer2 start         <preempted> writer3 start                     writer3 finishes                 writer2 finishes  writer1 finishesThis is very much like a writer being preempted by an interrupt andthe interrupt doing a write as well.Readers can happen at any time. But no two readers may run at thesame time, nor can a reader preempt/interrupt another reader. A readercannot preempt/interrupt a writer, but it may read/consume from thebuffer at the same time as a writer is writing, but the reader must beon another processor to do so. A reader may read on its own processorand can be preempted by a writer.A writer can preempt a reader, but a reader cannot preempt a writer.But a reader can read the buffer at the same time (on another processor)as a writer.The ring buffer is made up of a list of pages held together by a linked list.At initialization a reader page is allocated for the reader that is notpart of the ring buffer.The head_page, tail_page and commit_page are all initialized to pointto the same page.The reader page is initialized to have its next pointer pointing tothe head page, and its previous pointer pointing to a page beforethe head page.The reader has its own page to use. At start up time, this page isallocated but is not attached to the list. When the reader wantsto read from the buffer, if its page is empty (like it is on start-up),it will swap its page with the head_page. The old reader page willbecome part of the ring buffer and the head_page will be removed.The page after the inserted page (old reader_page) will become thenew head page.Once the new page is given to the reader, the reader could do whatit wants with it, as long as a writer has left that page.A sample of how the reader page is swapped: Note this does notshow the head page in the buffer, it is for demonstrating a swaponly.  +------+  |reader|          RING BUFFER  |page  |  +------+                  +---+   +---+   +---+                  |   |-->|   |-->|   |                  |   |<--|   |<--|   |                  +---+   +---+   +---+                   ^ |             ^ |                   | +-------------+ |                   +-----------------+  +------+  |reader|          RING BUFFER  |page  |-------------------+  +------+                   v    |             +---+   +---+   +---+    |             |   |-->|   |-->|   |    |             |   |<--|   |<--|   |<-+    |             +---+   +---+   +---+  |    |              ^ |             ^ |   |    |              | +-------------+ |   |    |              +-----------------+   |    +------------------------------------+  +------+  |reader|          RING BUFFER  |page  |-------------------+  +------+ <---------------+ v    |  ^          +---+   +---+   +---+    |  |          |   |-->|   |-->|   |    |  |          |   |   |   |<--|   |<-+    |  |          +---+   +---+   +---+  |    |  |             |             ^ |   |    |  |             +-------------+ |   |    |  +-----------------------------+   |    +------------------------------------+  +------+  |buffer|          RING BUFFER  |page  |-------------------+  +------+ <---------------+ v    |  ^          +---+   +---+   +---+    |  |          |   |   |   |-->|   |    |  |  New     |   |   |   |<--|   |<-+    |  | Reader   +---+   +---+   +---+  |    |  |  page ----^                 |   |    |  |                             |   |    |  +-----------------------------+   |    +------------------------------------+It is possible that the page swapped is the commit page and the tail page,if what is in the ring buffer is less than what is held in a buffer page.          reader page    commit page   tail page              |              |             |              v              |             |             +---+           |             |             |   |<----------+             |             |   |<------------------------+             |   |------+             +---+      |                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+This case is still valid for this algorithm.When the writer leaves the page, it simply goes into the ring buffersince the reader page still points to the next location in the ringbuffer.The main pointers:  reader page - The page used solely by the reader and is not part                of the ring buffer (may be swapped in)  head page - the next page in the ring buffer that will be swapped              with the reader page.  tail page - the page where the next write will take place.  commit page - the page that last finished a write.The commit page only is updated by the outermost writer in thewriter stack. A writer that preempts another writer will not move thecommit page.When data is written into the ring buffer, a position is reservedin the ring buffer and passed back to the writer. When the writeris finished writing data into that position, it commits the write.Another write (or a read) may take place at anytime during thistransaction. If another write happens it must finish before continuingwith the previous write.   Write reserve:       Buffer page      +---------+      |written  |      +---------+  <--- given back to writer (current commit)      |reserved |      +---------+ <--- tail pointer      | empty   |      +---------+   Write commit:       Buffer page      +---------+      |written  |      +---------+      |written  |      +---------+  <--- next position for write (current commit)      | empty   |      +---------+ If a write happens after the first reserve:       Buffer page      +---------+      |written  |      +---------+  <-- current commit      |reserved |      +---------+  <--- given back to second writer      |reserved |      +---------+ <--- tail pointer  After second writer commits:       Buffer page      +---------+      |written  |      +---------+  <--(last full commit)      |reserved |      +---------+      |pending  |      |commit   |      +---------+ <--- tail pointer  When the first writer commits:       Buffer page      +---------+      |written  |      +---------+      |written  |      +---------+      |written  |      +---------+  <--(last full commit and tail pointer)The commit pointer points to the last write location that wascommitted without preempting another write. When a write thatpreempted another write is committed, it only becomes a pending commitand will not be a full commit until all writes have been committed.The commit page points to the page that has the last full commit.The tail page points to the page with the last write (beforecommitting).The tail page is always equal to or after the commit page. It maybe several pages ahead. If the tail page catches up to the commitpage then no more writes may take place (regardless of the modeof the ring buffer: overwrite and produce/consumer).The order of pages is: head page commit page tail pagePossible scenario:                             tail page  head page         commit page  |      |                 |        |      v                 v        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+There is a special case that the head page is after either the commit pageand possibly the tail page. That is when the commit (and tail) page has beenswapped with the reader page. This is because the head page is alwayspart of the ring buffer, but the reader page is not. Whenever therehas been less than a full page that has been committed inside the ring buffer,and a reader swaps out a page, it will be swapping out the commit page.          reader page    commit page   tail page              |              |             |              v              |             |             +---+           |             |             |   |<----------+             |             |   |<------------------------+             |   |------+             +---+      |                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+                        ^                        |                    head pageIn this case, the head page will not move when the tail and commitmove back into the ring buffer.The reader cannot swap a page into the ring buffer if the commit pageis still on that page. If the read meets the last commit (real commitnot pending or reserved), then there is nothing more to read.The buffer is considered empty until another full commit finishes.When the tail meets the head page, if the buffer is in overwrite mode,the head page will be pushed ahead one. If the buffer is in producer/consumermode, the write will fail.Overwrite mode:            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+                        ^                        |                    head page            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+                                 ^                                 |                             head page                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+                                 ^                                 |                             head pageNote, the reader page will still point to the previous head page.But when a swap takes place, it will use the most recent head page.Making the Ring Buffer Lockless:--------------------------------The main idea behind the lockless algorithm is to combine the movingof the head_page pointer with the swapping of pages with the reader.State flags are placed inside the pointer to the page. To do this,each page must be aligned in memory by 4 bytes. This will allow the 2least significant bits of the address to be used as flags, sincethey will always be zero for the address. To get the address,simply mask out the flags.  MASK = ~3  address & MASKTwo flags will be kept by these two bits:   HEADER - the page being pointed to is a head page   UPDATE - the page being pointed to is being updated by a writer          and was or is about to be a head page.          reader page              |              v             +---+             |   |------+             +---+      |                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |-H->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The above pointer "-H->" would have the HEADER flag set. That isthe next page is the next page to be swapped out by the reader.This pointer means the next page is the head page.When the tail page meets the head pointer, it will use cmpxchg tochange the pointer to the UPDATE state:            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-H->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+"-U->" represents a pointer in the UPDATE state.Any access to the reader will need to take some sort of lock to serializethe readers. But the writers will never take a lock to write to thering buffer. This means we only need to worry about a single reader,and writes only preempt in "stack" formation.When the reader tries to swap the page with the ring buffer, itwill also use cmpxchg. If the flag bit in the pointer to thehead page does not have the HEADER flag set, the compare will failand the reader will need to look for the new head page and try again.Note, the flags UPDATE and HEADER are never set at the same time.The reader swaps the reader page as follows:  +------+  |reader|          RING BUFFER  |page  |  +------+                  +---+    +---+    +---+                  |   |--->|   |--->|   |                  |   |<---|   |<---|   |                  +---+    +---+    +---+                   ^ |               ^ |                   | +---------------+ |                   +-----H-------------+The reader sets the reader page next pointer as HEADER to the page afterthe head page.  +------+  |reader|          RING BUFFER  |page  |-------H-----------+  +------+                   v    |             +---+    +---+    +---+    |             |   |--->|   |--->|   |    |             |   |<---|   |<---|   |<-+    |             +---+    +---+    +---+  |    |              ^ |               ^ |   |    |              | +---------------+ |   |    |              +-----H-------------+   |    +--------------------------------------+It does a cmpxchg with the pointer to the previous head page to make itpoint to the reader page. Note that the new pointer does not have the HEADERflag set.  This action atomically moves the head page forward.  +------+  |reader|          RING BUFFER  |page  |-------H-----------+  +------+                   v    |  ^          +---+   +---+   +---+    |  |          |   |-->|   |-->|   |    |  |          |   |<--|   |<--|   |<-+    |  |          +---+   +---+   +---+  |    |  |             |             ^ |   |    |  |             +-------------+ |   |    |  +-----------------------------+   |    +------------------------------------+After the new head page is set, the previous pointer of the head page isupdated to the reader page.  +------+  |reader|          RING BUFFER  |page  |-------H-----------+  +------+ <---------------+ v    |  ^          +---+   +---+   +---+    |  |          |   |-->|   |-->|   |    |  |          |   |   |   |<--|   |<-+    |  |          +---+   +---+   +---+  |    |  |             |             ^ |   |    |  |             +-------------+ |   |    |  +-----------------------------+   |    +------------------------------------+  +------+  |buffer|          RING BUFFER  |page  |-------H-----------+  <--- New head page  +------+ <---------------+ v    |  ^          +---+   +---+   +---+    |  |          |   |   |   |-->|   |    |  |  New     |   |   |   |<--|   |<-+    |  | Reader   +---+   +---+   +---+  |    |  |  page ----^                 |   |    |  |                             |   |    |  +-----------------------------+   |    +------------------------------------+Another important point: The page that the reader page points back toby its previous pointer (the one that now points to the new head page)never points back to the reader page. That is because the reader page isnot part of the ring buffer. Traversing the ring buffer via the next pointerswill always stay in the ring buffer. Traversing the ring buffer via theprev pointers may not.Note, the way to determine a reader page is simply by examining the previouspointer of the page. If the next pointer of the previous page does notpoint back to the original page, then the original page is a reader page:             +--------+             | reader |  next   +----+             |  page  |-------->|    |<====== (buffer page)             +--------+         +----+                 |                | ^                 |                v | next            prev |              +----+                 +------------->|    |                                +----+The way the head page moves forward:When the tail page meets the head page and the buffer is in overwrite modeand more writes take place, the head page must be moved forward before thewriter may move the tail page. The way this is done is that the writerperforms a cmpxchg to convert the pointer to the head page from the HEADERflag to have the UPDATE flag set. Once this is done, the reader willnot be able to swap the head page from the buffer, nor will it be able tomove the head page, until the writer is finished with the move.This eliminates any races that the reader can have on the writer. The readermust spin, and this is why the reader cannot preempt the writer.            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-H->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The following page will be made into the new head page.           tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+After the new head page has been set, we can set the old head pagepointer back to NORMAL.           tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+After the head page has been moved, the tail page may now move forward.                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The above are the trivial updates. Now for the more complex scenarios.As stated before, if enough writes preempt the first write, thetail page may make it all the way around the buffer and meet the commitpage. At this time, we must start dropping writes (usually with some kindof warning to the user). But what happens if the commit was still on thereader page? The commit page is not part of the ring buffer. The tail pagemust account for this.          reader page    commit page              |              |              v              |             +---+           |             |   |<----------+             |   |             |   |------+             +---+      |                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |-H->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+               ^               |           tail pageIf the tail page were to simply push the head page forward, the commit whenleaving the reader page would not be pointing to the correct page.The solution to this is to test if the commit page is on the reader pagebefore pushing the head page. If it is, then it can be assumed that thetail page wrapped the buffer, and we must drop new writes.This is not a race condition, because the commit page can only be movedby the outermost writer (the writer that was preempted).This means that the commit will not move while a writer is moving thetail page. The reader cannot swap the reader page if it is also beingused as the commit page. The reader can simply check that the commitis off the reader page. Once the commit page leaves the reader pageit will never go back on it unless a reader does another swap with thebuffer page that is also the commit page.Nested writes-------------In the pushing forward of the tail page we must first push forwardthe head page if the head page is the next page. If the head pageis not the next page, the tail page is simply updated with a cmpxchg.Only writers move the tail page. This must be done atomically to protectagainst nested writers.  temp_page = tail_page  next_page = temp_page->next  cmpxchg(tail_page, temp_page, next_page)The above will update the tail page if it is still pointing to the expectedpage. If this fails, a nested write pushed it forward, the current writedoes not need to push it.           temp page               |               v            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+Nested write comes in and moves the tail page forward:                    tail page (moved by nested writer)            temp page   |               |        |               v        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The above would fail the cmpxchg, but since the tail page has alreadybeen moved forward, the writer will just try again to reserve storageon the new tail page.But the moving of the head page is a bit more complex.            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-H->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The write converts the head page pointer to UPDATE.            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+But if a nested writer preempts here, it will see that the nextpage is a head page, but it is also nested. It will detect thatit is nested and will save that information. The detection is thefact that it sees the UPDATE flag instead of a HEADER or NORMALpointer.The nested writer will set the new head page pointer.           tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+But it will not reset the update back to normal. Only the writerthat converted a pointer from HEAD to UPDATE will convert it backto NORMAL.                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+After the nested writer finishes, the outermost writer will convertthe UPDATE pointer to NORMAL.                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+It can be even more complex if several nested writes came in and movedthe tail page ahead several pages:(first writer)            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-H->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The write converts the head page pointer to UPDATE.            tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+Next writer comes in, and sees the update and sets up the newhead page.(second writer)           tail page               |               v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The nested writer moves the tail page forward. But does not set the oldupdate page to NORMAL because it is not the outermost writer.                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-H->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+Another writer preempts and sees the page after the tail page is a head page.It changes it from HEAD to UPDATE.(third writer)                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-U->|   |--->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The writer will move the head page forward:(third writer)                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-U->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+But now that the third writer did change the HEAD flag to UPDATE itwill convert it to normal:(third writer)                    tail page                        |                        v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+Then it will move the tail page, and return back to the second writer.(second writer)                             tail page                                 |                                 v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The second writer will fail to move the tail page because it was alreadymoved, so it will try again and add its data to the new tail page.It will return to the first writer.(first writer)                             tail page                                 |                                 v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+The first writer cannot know atomically if the tail page movedwhile it updates the HEAD page. It will then update the head page towhat it thinks is the new head page.(first writer)                             tail page                                 |                                 v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-H->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+Since the cmpxchg returns the old value of the pointer the first writerwill see it succeeded in updating the pointer from NORMAL to HEAD.But as we can see, this is not good enough. It must also check to seeif the tail page is either where it use to be or on the next page:(first writer)               A        B    tail page               |        |        |               v        v        v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |-H->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+If tail page != A and tail page != B, then it must reset the pointerback to NORMAL. The fact that it only needs to worry about nestedwriters means that it only needs to check this after setting the HEAD page.(first writer)               A        B    tail page               |        |        |               v        v        v    +---+    +---+    +---+    +---+<---|   |--->|   |-U->|   |--->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+Now the writer can update the head page. This is also why the head page mustremain in UPDATE and only reset by the outermost writer. This preventsthe reader from seeing the incorrect head page.(first writer)               A        B    tail page               |        |        |               v        v        v    +---+    +---+    +---+    +---+<---|   |--->|   |--->|   |--->|   |-H->--->|   |<---|   |<---|   |<---|   |<---    +---+    +---+    +---+    +---+