ring-buffer-design.rar_V2 _buffer_ring_buffer

Lockless Ring Buffer Design =========================== Copyright 2009 Red Hat Inc. Author: Steven Rostedt <[email protected]> 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: 2.6.31 Terminology 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 producer consumer - 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 next tail_page - a pointer to the page that will be written to next commit_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 in producer/consumer mode. Producer/consumer mode is where if the producer were to fill up the buffer before the consumer could free up anything, the producer will stop writing to the buffer. This will lose most recent events. Overwrite mode is where if the producer were to fill up the buffer before the consumer could free up anything, the producer will overwrite 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 writing before the previous writer may continue. This is very important to the algorithm. The writers act like a "stack". The way interrupts works enforces this behavior. writer1 start <preempted> writer2 start <preempted> writer3 start writer3 finishes writer2 finishes writer1 finishes This is very much like a writer being preempted by an interrupt and the interrupt doing a write as well. Readers can happen at any time. But no two readers may run at the same time, nor can a reader preempt/interrupt another reader. A reader cannot preempt/interrupt a writer, but it may read/consume from the buffer at the same time as a writer is writing, but the reader must be on another processor to do so. A reader may read on its own processor and 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 not part of the ring buffer. The head_page, tail_page and commit_page are all initialized to point to the same page. The reader page is initialized to have its next pointer pointing to the head page, and its previous pointer pointing to a page before the head page. The reader has its own page to use. At start up time, this page is allocated but is not attached to the list. When the reader wants to 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 will become part of the ring buffer and the head_page will be removed. The page after the inserted page (old reader_page) will become the new head page. Once the new page is given to the reader, the reader could do what it wants with it, as long as a writer has left that page. A sample of how the reader page is swapped: Note this does not show the head page in the buffer, it is for demonstrating a swap only. +------+ |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 buffer since the reader page still points to the next location in the ring buffer. 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 the writer stack. A writer that preempts another writer will not move the commit page. When data is written into the ring buffer, a position is reserved in the ring buffer and passed back to the writer. When the writer is finished writing data into that position, it commits the write. Another write (or a read) may take place at anytime during this transaction. If another write happens it must finish before continuing with 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)