mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-21 19:53:59 +08:00
25985edced
Fixes generated by 'codespell' and manually reviewed. Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
956 lines
30 KiB
Plaintext
956 lines
30 KiB
Plaintext
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: 2.6.31
|
|
|
|
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)
|
|
| 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 was
|
|
committed without preempting another write. When a write that
|
|
preempted another write is committed, it only becomes a pending commit
|
|
and 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 (before
|
|
committing).
|
|
|
|
The tail page is always equal to or after the commit page. It may
|
|
be several pages ahead. If the tail page catches up to the commit
|
|
page then no more writes may take place (regardless of the mode
|
|
of the ring buffer: overwrite and produce/consumer).
|
|
|
|
The order of pages is:
|
|
|
|
head page
|
|
commit page
|
|
tail page
|
|
|
|
Possible scenario:
|
|
tail page
|
|
head page commit page |
|
|
| | |
|
|
v v v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |--->| |--->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
|
|
There is a special case that the head page is after either the commit page
|
|
and possibly the tail page. That is when the commit (and tail) page has been
|
|
swapped with the reader page. This is because the head page is always
|
|
part of the ring buffer, but the reader page is not. Whenever there
|
|
has 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 page
|
|
|
|
|
|
In this case, the head page will not move when the tail and commit
|
|
move back into the ring buffer.
|
|
|
|
The reader cannot swap a page into the ring buffer if the commit page
|
|
is still on that page. If the read meets the last commit (real commit
|
|
not 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/consumer
|
|
mode, the write will fail.
|
|
|
|
Overwrite mode:
|
|
|
|
tail page
|
|
|
|
|
v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |--->| |--->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
^
|
|
|
|
|
head page
|
|
|
|
|
|
tail page
|
|
|
|
|
v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |--->| |--->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
^
|
|
|
|
|
head page
|
|
|
|
|
|
tail page
|
|
|
|
|
v
|
|
+---+ +---+ +---+ +---+
|
|
<---| |--->| |--->| |--->| |--->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
^
|
|
|
|
|
head page
|
|
|
|
Note, 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 moving
|
|
of 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 2
|
|
least significant bits of the address to be used as flags, since
|
|
they will always be zero for the address. To get the address,
|
|
simply mask out the flags.
|
|
|
|
MASK = ~3
|
|
|
|
address & MASK
|
|
|
|
Two 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 is
|
|
the 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 to
|
|
change 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 serialize
|
|
the readers. But the writers will never take a lock to write to the
|
|
ring 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, it
|
|
will also use cmpxchg. If the flag bit in the pointer to the
|
|
head page does not have the HEADER flag set, the compare will fail
|
|
and 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 after
|
|
the head page.
|
|
|
|
|
|
+------+
|
|
|reader| RING BUFFER
|
|
|page |-------H-----------+
|
|
+------+ v
|
|
| +---+ +---+ +---+
|
|
| | |--->| |--->| |
|
|
| | |<---| |<---| |<-+
|
|
| +---+ +---+ +---+ |
|
|
| ^ | ^ | |
|
|
| | +---------------+ | |
|
|
| +-----H-------------+ |
|
|
+--------------------------------------+
|
|
|
|
It does a cmpxchg with the pointer to the previous head page to make it
|
|
point to the reader page. Note that the new pointer does not have the HEADER
|
|
flag 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 is
|
|
updated 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 to
|
|
by 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 is
|
|
not part of the ring buffer. Traversing the ring buffer via the next pointers
|
|
will always stay in the ring buffer. Traversing the ring buffer via the
|
|
prev pointers may not.
|
|
|
|
Note, the way to determine a reader page is simply by examining the previous
|
|
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 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->
|
|
--->| |<---| |<---| |<---| |<---
|
|
+---+ +---+ +---+ +---+
|
|
|