984 lines
31 KiB
ReStructuredText
984 lines
31 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.2-no-invariants-only
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===========================
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Lockless Ring Buffer Design
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===========================
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Copyright 2009 Red Hat Inc.
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:Author: Steven Rostedt <srostedt@redhat.com>
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:License: The GNU Free Documentation License, Version 1.2
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(dual licensed under the GPL v2)
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:Reviewers: Mathieu Desnoyers, Huang Ying, Hidetoshi Seto,
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and Frederic Weisbecker.
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Written for: 2.6.31
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Terminology used in this Document
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---------------------------------
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tail
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- where new writes happen in the ring buffer.
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head
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- where new reads happen in the ring buffer.
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producer
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- the task that writes into the ring buffer (same as writer)
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writer
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- same as producer
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consumer
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- the task that reads from the buffer (same as reader)
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reader
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- same as consumer.
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reader_page
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- A page outside the ring buffer used solely (for the most part)
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by the reader.
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head_page
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- a pointer to the page that the reader will use next
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tail_page
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- a pointer to the page that will be written to next
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commit_page
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- a pointer to the page with the last finished non-nested write.
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cmpxchg
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- hardware-assisted atomic transaction that performs the following::
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A = B if previous A == C
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R = cmpxchg(A, C, B) is saying that we replace A with B if and only
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if current A is equal to C, and we put the old (current)
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A into R
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R gets the previous A regardless if A is updated with B or not.
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To see if the update was successful a compare of ``R == C``
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may be used.
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The Generic Ring Buffer
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-----------------------
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The ring buffer can be used in either an overwrite mode or in
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producer/consumer mode.
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Producer/consumer mode is where if the producer were to fill up the
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buffer before the consumer could free up anything, the producer
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will stop writing to the buffer. This will lose most recent events.
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Overwrite mode is where if the producer were to fill up the buffer
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before the consumer could free up anything, the producer will
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overwrite the older data. This will lose the oldest events.
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No two writers can write at the same time (on the same per-cpu buffer),
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but a writer may interrupt another writer, but it must finish writing
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before the previous writer may continue. This is very important to the
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algorithm. The writers act like a "stack". The way interrupts works
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enforces this behavior::
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writer1 start
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<preempted> writer2 start
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<preempted> writer3 start
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writer3 finishes
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writer2 finishes
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writer1 finishes
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This is very much like a writer being preempted by an interrupt and
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the interrupt doing a write as well.
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Readers can happen at any time. But no two readers may run at the
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same time, nor can a reader preempt/interrupt another reader. A reader
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cannot preempt/interrupt a writer, but it may read/consume from the
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buffer at the same time as a writer is writing, but the reader must be
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on another processor to do so. A reader may read on its own processor
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and can be preempted by a writer.
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A writer can preempt a reader, but a reader cannot preempt a writer.
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But a reader can read the buffer at the same time (on another processor)
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as a writer.
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The ring buffer is made up of a list of pages held together by a linked list.
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At initialization a reader page is allocated for the reader that is not
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part of the ring buffer.
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The head_page, tail_page and commit_page are all initialized to point
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to the same page.
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The reader page is initialized to have its next pointer pointing to
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the head page, and its previous pointer pointing to a page before
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the head page.
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The reader has its own page to use. At start up time, this page is
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allocated but is not attached to the list. When the reader wants
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to read from the buffer, if its page is empty (like it is on start-up),
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it will swap its page with the head_page. The old reader page will
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become part of the ring buffer and the head_page will be removed.
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The page after the inserted page (old reader_page) will become the
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new head page.
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Once the new page is given to the reader, the reader could do what
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it wants with it, as long as a writer has left that page.
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A sample of how the reader page is swapped: Note this does not
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show the head page in the buffer, it is for demonstrating a swap
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only.
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::
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+------+
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|reader| RING BUFFER
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|page |
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+------+
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+---+ +---+ +---+
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| |-->| |-->| |
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| |<--| |<--| |
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+---+ +---+ +---+
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^ | ^ |
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| +-------------+ |
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+-----------------+
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+------+
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|reader| RING BUFFER
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|page |-------------------+
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+------+ v
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| +---+ +---+ +---+
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| | |-->| |-->| |
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| | |<--| |<--| |<-+
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| +---+ +---+ +---+ |
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| ^ | ^ | |
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| | +-------------+ | |
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| +-----------------+ |
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+------------------------------------+
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+------+
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|reader| RING BUFFER
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|page |-------------------+
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+------+ <---------------+ v
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| ^ +---+ +---+ +---+
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| | | |-->| |-->| |
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| | | | | |<--| |<-+
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| | +---+ +---+ +---+ |
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| | | ^ | |
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| | +-------------+ | |
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| +-----------------------------+ |
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+------------------------------------+
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+------+
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|buffer| RING BUFFER
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|page |-------------------+
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+------+ <---------------+ v
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| ^ +---+ +---+ +---+
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| | | | | |-->| |
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| | New | | | |<--| |<-+
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| | Reader +---+ +---+ +---+ |
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| | page ----^ | |
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| | | |
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| +-----------------------------+ |
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+------------------------------------+
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It is possible that the page swapped is the commit page and the tail page,
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if what is in the ring buffer is less than what is held in a buffer page.
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::
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reader page commit page tail page
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v | |
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+---+ | |
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| |<----------+ |
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| |<------------------------+
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| |------+
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+---+ |
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |--->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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This case is still valid for this algorithm.
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When the writer leaves the page, it simply goes into the ring buffer
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since the reader page still points to the next location in the ring
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buffer.
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The main pointers:
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reader page
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- The page used solely by the reader and is not part
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of the ring buffer (may be swapped in)
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head page
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- the next page in the ring buffer that will be swapped
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with the reader page.
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tail page
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- the page where the next write will take place.
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commit page
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- the page that last finished a write.
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The commit page only is updated by the outermost writer in the
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writer stack. A writer that preempts another writer will not move the
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commit page.
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When data is written into the ring buffer, a position is reserved
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in the ring buffer and passed back to the writer. When the writer
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is finished writing data into that position, it commits the write.
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Another write (or a read) may take place at anytime during this
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transaction. If another write happens it must finish before continuing
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with the previous write.
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Write reserve::
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Buffer page
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+---------+
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|written |
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+---------+ <--- given back to writer (current commit)
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|reserved |
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+---------+ <--- tail pointer
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| empty |
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+---------+
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Write commit::
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Buffer page
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+---------+
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|written |
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+---------+
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|written |
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+---------+ <--- next position for write (current commit)
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| empty |
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+---------+
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If a write happens after the first reserve::
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Buffer page
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+---------+
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|written |
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+---------+ <-- current commit
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|reserved |
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+---------+ <--- given back to second writer
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|reserved |
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+---------+ <--- tail pointer
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After second writer commits::
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Buffer page
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+---------+
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|written |
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+---------+ <--(last full commit)
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|reserved |
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+---------+
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|pending |
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|commit |
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+---------+ <--- tail pointer
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When the first writer commits::
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Buffer page
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+---------+
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|written |
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+---------+
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|written |
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+---------+
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|written |
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+---------+ <--(last full commit and tail pointer)
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The commit pointer points to the last write location that was
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committed without preempting another write. When a write that
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preempted another write is committed, it only becomes a pending commit
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and will not be a full commit until all writes have been committed.
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The commit page points to the page that has the last full commit.
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The tail page points to the page with the last write (before
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committing).
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The tail page is always equal to or after the commit page. It may
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be several pages ahead. If the tail page catches up to the commit
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page then no more writes may take place (regardless of the mode
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of the ring buffer: overwrite and produce/consumer).
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The order of pages is::
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head page
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commit page
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tail page
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Possible scenario::
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tail page
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head page commit page |
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v v v
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+---+ +---+ +---+ +---+
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<---| |--->| |--->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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There is a special case that the head page is after either the commit page
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and possibly the tail page. That is when the commit (and tail) page has been
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swapped with the reader page. This is because the head page is always
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part of the ring buffer, but the reader page is not. Whenever there
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has been less than a full page that has been committed inside the ring buffer,
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and a reader swaps out a page, it will be swapping out the commit page.
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::
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reader page commit page tail page
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v | |
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+---+ | |
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| |<----------+ |
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| |<------------------------+
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| |------+
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+---+ |
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |--->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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^
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head page
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In this case, the head page will not move when the tail and commit
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move back into the ring buffer.
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The reader cannot swap a page into the ring buffer if the commit page
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is still on that page. If the read meets the last commit (real commit
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not pending or reserved), then there is nothing more to read.
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The buffer is considered empty until another full commit finishes.
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When the tail meets the head page, if the buffer is in overwrite mode,
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the head page will be pushed ahead one. If the buffer is in producer/consumer
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mode, the write will fail.
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Overwrite mode::
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tail page
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |--->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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^
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head page
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tail page
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |--->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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^
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head page
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tail page
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |--->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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^
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head page
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Note, the reader page will still point to the previous head page.
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But when a swap takes place, it will use the most recent head page.
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Making the Ring Buffer Lockless:
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--------------------------------
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The main idea behind the lockless algorithm is to combine the moving
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of the head_page pointer with the swapping of pages with the reader.
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State flags are placed inside the pointer to the page. To do this,
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each page must be aligned in memory by 4 bytes. This will allow the 2
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least significant bits of the address to be used as flags, since
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they will always be zero for the address. To get the address,
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simply mask out the flags::
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MASK = ~3
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address & MASK
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Two flags will be kept by these two bits:
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HEADER
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- the page being pointed to is a head page
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UPDATE
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- the page being pointed to is being updated by a writer
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and was or is about to be a head page.
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::
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reader page
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v
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+---+
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| |------+
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+---+ |
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |-H->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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The above pointer "-H->" would have the HEADER flag set. That is
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the next page is the next page to be swapped out by the reader.
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This pointer means the next page is the head page.
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When the tail page meets the head pointer, it will use cmpxchg to
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change the pointer to the UPDATE state::
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tail page
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |-H->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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tail page
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v
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+---+ +---+ +---+ +---+
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<---| |--->| |-U->| |--->| |--->
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--->| |<---| |<---| |<---| |<---
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+---+ +---+ +---+ +---+
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"-U->" represents a pointer in the UPDATE state.
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Any access to the reader will need to take some sort of lock to serialize
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the readers. But the writers will never take a lock to write to the
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ring buffer. This means we only need to worry about a single reader,
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and writes only preempt in "stack" formation.
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When the reader tries to swap the page with the ring buffer, it
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will also use cmpxchg. If the flag bit in the pointer to the
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head page does not have the HEADER flag set, the compare will fail
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and the reader will need to look for the new head page and try again.
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Note, the flags UPDATE and HEADER are never set at the same time.
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The reader swaps the reader page as follows::
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+------+
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|reader| RING BUFFER
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|page |
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+------+
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+---+ +---+ +---+
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| |--->| |--->| |
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| |<---| |<---| |
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+---+ +---+ +---+
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^ | ^ |
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| +---------------+ |
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+-----H-------------+
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The reader sets the reader page next pointer as HEADER to the page after
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the head page::
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+------+
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|reader| RING BUFFER
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|page |-------H-----------+
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+------+ v
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| +---+ +---+ +---+
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| | |--->| |--->| |
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| | |<---| |<---| |<-+
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| +---+ +---+ +---+ |
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| ^ | ^ | |
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| | +---------------+ | |
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| +-----H-------------+ |
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+--------------------------------------+
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It does a cmpxchg with the pointer to the previous head page to make it
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point to the reader page. Note that the new pointer does not have the HEADER
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flag set. This action atomically moves the head page forward::
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+------+
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|reader| RING BUFFER
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|page |-------H-----------+
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+------+ v
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| ^ +---+ +---+ +---+
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| | | |-->| |-->| |
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| | | |<--| |<--| |<-+
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| | +---+ +---+ +---+ |
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| | | ^ | |
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| | +-------------+ | |
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| +-----------------------------+ |
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+------------------------------------+
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After the new head page is set, the previous pointer of the head page is
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updated to the reader page::
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+------+
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|reader| RING BUFFER
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|page |-------H-----------+
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+------+ <---------------+ v
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| ^ +---+ +---+ +---+
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| | | |-->| |-->| |
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| | | | | |<--| |<-+
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| | +---+ +---+ +---+ |
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| | | ^ | |
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| | +-------------+ | |
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| +-----------------------------+ |
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+------------------------------------+
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+------+
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|buffer| RING BUFFER
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|page |-------H-----------+ <--- New head page
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+------+ <---------------+ v
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| ^ +---+ +---+ +---+
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| | | | | |-->| |
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| | New | | | |<--| |<-+
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| | Reader +---+ +---+ +---+ |
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| | page ----^ | |
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| | | |
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| +-----------------------------+ |
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+------------------------------------+
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Another important point: The page that the reader page points back to
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by its previous pointer (the one that now points to the new head page)
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never points back to the reader page. That is because the reader page is
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not part of the ring buffer. Traversing the ring buffer via the next pointers
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will always stay in the ring buffer. Traversing the ring buffer via the
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prev pointers may not.
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Note, the way to determine a reader page is simply by examining the previous
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|
pointer of the page. If the next pointer of the previous page does not
|
|
point 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 mode
|
|
and more writes take place, the head page must be moved forward before the
|
|
writer may move the tail page. The way this is done is that the writer
|
|
performs a cmpxchg to convert the pointer to the head page from the HEADER
|
|
flag to have the UPDATE flag set. Once this is done, the reader will
|
|
not be able to swap the head page from the buffer, nor will it be able to
|
|
move the head page, until the writer is finished with the move.
|
|
|
|
This eliminates any races that the reader can have on the writer. The reader
|
|
must 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 page
|
|
pointer 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, the
|
|
tail page may make it all the way around the buffer and meet the commit
|
|
page. At this time, we must start dropping writes (usually with some kind
|
|
of warning to the user). But what happens if the commit was still on the
|
|
reader page? The commit page is not part of the ring buffer. The tail page
|
|
must account for this::
|
|
|
|
|
|
reader page commit page
|
|
| |
|
|
v |
|
|
+---+ |
|
|
| |<----------+
|
|
| |
|
|
| |------+
|
|
+---+ |
|
|
|
|
|
v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |-H->| |--->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
^
|
|
|
|
|
tail page
|
|
|
|
If the tail page were to simply push the head page forward, the commit when
|
|
leaving 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 page
|
|
before pushing the head page. If it is, then it can be assumed that the
|
|
tail page wrapped the buffer, and we must drop new writes.
|
|
|
|
This is not a race condition, because the commit page can only be moved
|
|
by the outermost writer (the writer that was preempted).
|
|
This means that the commit will not move while a writer is moving the
|
|
tail page. The reader cannot swap the reader page if it is also being
|
|
used as the commit page. The reader can simply check that the commit
|
|
is off the reader page. Once the commit page leaves the reader page
|
|
it will never go back on it unless a reader does another swap with the
|
|
buffer page that is also the commit page.
|
|
|
|
|
|
Nested writes
|
|
-------------
|
|
|
|
In the pushing forward of the tail page we must first push forward
|
|
the head page if the head page is the next page. If the head page
|
|
is 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 protect
|
|
against 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 expected
|
|
page. If this fails, a nested write pushed it forward, the current write
|
|
does 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 already
|
|
been moved forward, the writer will just try again to reserve storage
|
|
on 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 next
|
|
page is a head page, but it is also nested. It will detect that
|
|
it is nested and will save that information. The detection is the
|
|
fact that it sees the UPDATE flag instead of a HEADER or NORMAL
|
|
pointer.
|
|
|
|
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 writer
|
|
that converted a pointer from HEAD to UPDATE will convert it back
|
|
to NORMAL::
|
|
|
|
tail page
|
|
|
|
|
v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |-U->| |-H->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
|
|
After the nested writer finishes, the outermost writer will convert
|
|
the UPDATE pointer to NORMAL::
|
|
|
|
|
|
tail page
|
|
|
|
|
v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |--->| |-H->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
|
|
|
|
It can be even more complex if several nested writes came in and moved
|
|
the 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 new
|
|
head page::
|
|
|
|
(second writer)
|
|
|
|
tail page
|
|
|
|
|
v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |-U->| |-H->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
|
|
The nested writer moves the tail page forward. But does not set the old
|
|
update 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 it
|
|
will 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 already
|
|
moved, 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 moved
|
|
while it updates the HEAD page. It will then update the head page to
|
|
what 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 writer
|
|
will 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 see
|
|
if 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 pointer
|
|
back to NORMAL. The fact that it only needs to worry about nested
|
|
writers 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 must
|
|
remain in UPDATE and only reset by the outermost writer. This prevents
|
|
the reader from seeing the incorrect head page::
|
|
|
|
|
|
(first writer)
|
|
|
|
A B tail page
|
|
| | |
|
|
v v v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |--->| |--->| |-H->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|