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.
+commit_page - a pointer to the page with the last finished non-nested write.
-cmpxchg - hardware assisted atomic transaction that performs the following:
+cmpxchg - hardware-assisted atomic transaction that performs the following:
A = B iff previous A == C
The ring buffer can be used in either an overwrite mode or in
producer/consumer mode.
-Producer/consumer mode is where the producer were to fill up the
+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 the produce were to fill up the buffer
+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),
+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
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
-can not preempt/interrupt a writer, but it may read/consume from the
+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 can not preempt 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 link list.
+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 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)
+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
commit page - the page that last finished a write.
-The commit page only is updated by the outer most writer in the
+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.
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 till all writes have been committed.
+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
page then no more writes may take place (regardless of the mode
of the ring buffer: overwrite and produce/consumer).
-The order of pages are:
+The order of pages is:
head page
commit page
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. When ever there
+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.
In this case, the head page will not move when the tail and commit
move back into the ring buffer.
-The reader can not swap a page into the ring buffer if the commit page
+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.
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
+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.
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 flag UPDATE and HEADER are never set at the same time.
+Note, the flags UPDATE and HEADER are never set at the same time.
The reader swaps the reader page as follows:
| +-----------------------------+ |
+------------------------------------+
-Another important point. The page that the reader page points back to
+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
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 can not preempt the writer.
+must spin, and this is why the reader cannot preempt the writer.
tail page
|
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 outter most writer (the writer that was preempted).
+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 can not swap the reader page if it is also being
+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
--->| |<---| |<---| |<---| |<---
+---+ +---+ +---+ +---+
-But if a nested writer preempts here. It will see that the next
+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
--->| |<---| |<---| |<---| |<---
+---+ +---+ +---+ +---+
-After the nested writer finishes, the outer most writer will convert
+After the nested writer finishes, the outermost writer will convert
the UPDATE pointer to NORMAL.
+---+ +---+ +---+ +---+
The nested writer moves the tail page forward. But does not set the old
-update page to NORMAL because it is not the outer most writer.
+update page to NORMAL because it is not the outermost writer.
tail page
|
--->| |<---| |<---| |<---| |<---
+---+ +---+ +---+ +---+
-The first writer can not know atomically test if the tail page moved
+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.
--->| |<---| |<---| |<---| |<---
+---+ +---+ +---+ +---+
-If tail page != A and tail page does not equal B, then it must reset the
-pointer back to NORMAL. The fact that it only needs to worry about
-nested writers, it only needs to check this after setting the HEAD page.
+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)
+---+ +---+ +---+ +---+
Now the writer can update the head page. This is also why the head page must
-remain in UPDATE and only reset by the outer most writer. This prevents
+remain in UPDATE and only reset by the outermost writer. This prevents
the reader from seeing the incorrect head page.
git://ftp.safe.ca / safe / jmp / linux-2.6 / commitdiff