Merge branch 'tracing-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'tracing-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip: (105 commits)
  ring-buffer: only enable ring_buffer_swap_cpu when needed
  ring-buffer: check for swapped buffers in start of committing
  tracing: report error in trace if we fail to swap latency buffer
  tracing: add trace_array_printk for internal tracers to use
  tracing: pass around ring buffer instead of tracer
  tracing: make tracing_reset safe for external use
  tracing: use timestamp to determine start of latency traces
  tracing: Remove mentioning of legacy latency_trace file from documentation
  tracing/filters: Defer pred allocation, fix memory leak
  tracing: remove users of tracing_reset
  tracing: disable buffers and synchronize_sched before resetting
  tracing: disable update max tracer while reading trace
  tracing: print out start and stop in latency traces
  ring-buffer: disable all cpu buffers when one finds a problem
  ring-buffer: do not count discarded events
  ring-buffer: remove ring_buffer_event_discard
  ring-buffer: fix ring_buffer_read crossing pages
  ring-buffer: remove unnecessary cpu_relax
  ring-buffer: do not swap buffers during a commit
  ring-buffer: do not reset while in a commit
  ...

5 files changed, 1047 insertions, 32 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 7936b80..8e91863 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2480,6 +2480,11 @@ and is between 256 and 4096 characters. It is defined in the file
 	trace_buf_size=nn[KMG]
 			[FTRACE] will set tracing buffer size.
 
+	trace_event=[event-list]
+			[FTRACE] Set and start specified trace events in order
+			to facilitate early boot debugging.
+			See also Documentation/trace/events.txt
+
 	trix=		[HW,OSS] MediaTrix AudioTrix Pro
 			Format:
 			<io>,<irq>,<dma>,<dma2>,<sb_io>,<sb_irq>,<sb_dma>,<mpu_io>,<mpu_irq>
diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt
index f157d75..2bcc8d4 100644
--- a/Documentation/trace/events.txt
+++ b/Documentation/trace/events.txt
@@ -83,6 +83,15 @@ When reading one of these enable files, there are four results:
  X - there is a mixture of events enabled and disabled
  ? - this file does not affect any event
 
+2.3 Boot option
+---------------
+
+In order to facilitate early boot debugging, use boot option:
+
+	trace_event=[event-list]
+
+The format of this boot option is the same as described in section 2.1.
+
 3. Defining an event-enabled tracepoint
 =======================================
 
diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
index a39b3c7..355d0f1 100644
--- a/Documentation/trace/ftrace.txt
+++ b/Documentation/trace/ftrace.txt
@@ -85,26 +85,19 @@ of ftrace. Here is a list of some of the key files:
 	This file holds the output of the trace in a human
 	readable format (described below).
 
-  latency_trace:
-
-	This file shows the same trace but the information
-	is organized more to display possible latencies
-	in the system (described below).
-
   trace_pipe:
 
 	The output is the same as the "trace" file but this
 	file is meant to be streamed with live tracing.
-	Reads from this file will block until new data
-	is retrieved. Unlike the "trace" and "latency_trace"
-	files, this file is a consumer. This means reading
-	from this file causes sequential reads to display
-	more current data. Once data is read from this
-	file, it is consumed, and will not be read
-	again with a sequential read. The "trace" and
-	"latency_trace" files are static, and if the
-	tracer is not adding more data, they will display
-	the same information every time they are read.
+	Reads from this file will block until new data is
+	retrieved.  Unlike the "trace" file, this file is a
+	consumer. This means reading from this file causes
+	sequential reads to display more current data. Once
+	data is read from this file, it is consumed, and
+	will not be read again with a sequential read. The
+	"trace" file is static, and if the tracer is not
+	adding more data,they will display the same
+	information every time they are read.
 
   trace_options:
 
@@ -117,10 +110,10 @@ of ftrace. Here is a list of some of the key files:
 	Some of the tracers record the max latency.
 	For example, the time interrupts are disabled.
 	This time is saved in this file. The max trace
-	will also be stored, and displayed by either
-	"trace" or "latency_trace".  A new max trace will
-	only be recorded if the latency is greater than
-	the value in this file. (in microseconds)
+	will also be stored, and displayed by "trace".
+	A new max trace will only be recorded if the
+	latency is greater than the value in this
+	file. (in microseconds)
 
   buffer_size_kb:
 
@@ -210,7 +203,7 @@ Here is the list of current tracers that may be configured.
 	the trace with the longest max latency.
 	See tracing_max_latency. When a new max is recorded,
 	it replaces the old trace. It is best to view this
-	trace via the latency_trace file.
+	trace with the latency-format option enabled.
 
   "preemptoff"
 
@@ -307,8 +300,8 @@ the lowest priority thread (pid 0).
 Latency trace format
 --------------------
 
-For traces that display latency times, the latency_trace file
-gives somewhat more information to see why a latency happened.
+When the latency-format option is enabled, the trace file gives
+somewhat more information to see why a latency happened.
 Here is a typical trace.
 
 # tracer: irqsoff
@@ -380,9 +373,10 @@ explains which is which.
 
 The above is mostly meaningful for kernel developers.
 
-  time: This differs from the trace file output. The trace file output
-	includes an absolute timestamp. The timestamp used by the
-	latency_trace file is relative to the start of the trace.
+  time: When the latency-format option is enabled, the trace file
+	output includes a timestamp relative to the start of the
+	trace. This differs from the output when latency-format
+	is disabled, which includes an absolute timestamp.
 
   delay: This is just to help catch your eye a bit better. And
 	 needs to be fixed to be only relative to the same CPU.
@@ -440,7 +434,8 @@ Here are the available options:
   sym-addr:
    bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
 
-  verbose - This deals with the latency_trace file.
+  verbose - This deals with the trace file when the
+            latency-format option is enabled.
 
     bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
     (+0.000ms): simple_strtoul (strict_strtoul)
@@ -472,7 +467,7 @@ Here are the available options:
 		the app is no longer running
 
 		The lookup is performed when you read
-		trace,trace_pipe,latency_trace. Example:
+		trace,trace_pipe. Example:
 
 		a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
@@ -481,6 +476,11 @@ x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
 	       every scheduling event. Will add overhead if
 	       there's a lot of tasks running at once.
 
+  latency-format - This option changes the trace. When
+                   it is enabled, the trace displays
+                   additional information about the
+                   latencies, as described in "Latency
+                   trace format".
 
 sched_switch
 ------------
@@ -596,12 +596,13 @@ To reset the maximum, echo 0 into tracing_max_latency. Here is
 an example:
 
  # echo irqsoff > current_tracer
+ # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_enabled
  # ls -ltr
  [...]
  # echo 0 > tracing_enabled
- # cat latency_trace
+ # cat trace
 # tracer: irqsoff
 #
 irqsoff latency trace v1.1.5 on 2.6.26
@@ -703,12 +704,13 @@ which preemption was disabled. The control of preemptoff tracer
 is much like the irqsoff tracer.
 
  # echo preemptoff > current_tracer
+ # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_enabled
  # ls -ltr
  [...]
  # echo 0 > tracing_enabled
- # cat latency_trace
+ # cat trace
 # tracer: preemptoff
 #
 preemptoff latency trace v1.1.5 on 2.6.26-rc8
@@ -850,12 +852,13 @@ Again, using this trace is much like the irqsoff and preemptoff
 tracers.
 
  # echo preemptirqsoff > current_tracer
+ # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_enabled
  # ls -ltr
  [...]
  # echo 0 > tracing_enabled
- # cat latency_trace
+ # cat trace
 # tracer: preemptirqsoff
 #
 preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
@@ -1012,11 +1015,12 @@ Instead of performing an 'ls', we will run 'sleep 1' under
 'chrt' which changes the priority of the task.
 
  # echo wakeup > current_tracer
+ # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_enabled
  # chrt -f 5 sleep 1
  # echo 0 > tracing_enabled
- # cat latency_trace
+ # cat trace
 # tracer: wakeup
 #
 wakeup latency trace v1.1.5 on 2.6.26-rc8
diff --git a/Documentation/trace/function-graph-fold.vim b/Documentation/trace/function-graph-fold.vim
new file mode 100644
index 0000000..0544b50
--- /dev/null
+++ b/Documentation/trace/function-graph-fold.vim
@@ -0,0 +1,42 @@
+" Enable folding for ftrace function_graph traces.
+"
+" To use, :source this file while viewing a function_graph trace, or use vim's
+" -S option to load from the command-line together with a trace.  You can then
+" use the usual vim fold commands, such as "za", to open and close nested
+" functions.  While closed, a fold will show the total time taken for a call,
+" as would normally appear on the line with the closing brace.  Folded
+" functions will not include finish_task_switch(), so folding should remain
+" relatively sane even through a context switch.
+"
+" Note that this will almost certainly only work well with a
+" single-CPU trace (e.g. trace-cmd report --cpu 1).
+
+function! FunctionGraphFoldExpr(lnum)
+  let line = getline(a:lnum)
+  if line[-1:] == '{'
+    if line =~ 'finish_task_switch() {$'
+      return '>1'
+    endif
+    return 'a1'
+  elseif line[-1:] == '}'
+    return 's1'
+  else
+    return '='
+  endif
+endfunction
+
+function! FunctionGraphFoldText()
+  let s = split(getline(v:foldstart), '|', 1)
+  if getline(v:foldend+1) =~ 'finish_task_switch() {$'
+    let s[2] = ' task switch  '
+  else
+    let e = split(getline(v:foldend), '|', 1)
+    let s[2] = e[2]
+  endif
+  return join(s, '|')
+endfunction
+
+setlocal foldexpr=FunctionGraphFoldExpr(v:lnum)
+setlocal foldtext=FunctionGraphFoldText()
+setlocal foldcolumn=12
+setlocal foldmethod=expr
diff --git a/Documentation/trace/ring-buffer-design.txt b/Documentation/trace/ring-buffer-design.txt
new file mode 100644
index 0000000..5b1d23d
--- /dev/null
+++ b/Documentation/trace/ring-buffer-design.txt
@@ -0,0 +1,955 @@
+		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 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
+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
+can not 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.
+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.
+
+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 outer most 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 positon 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 till 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 are:
+
+ 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. When ever 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 can not 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 flag 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 can not 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 outter most 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
+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 outer most 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 outer most 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 can not know atomically test 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 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.
+
+
+(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 outer most writer. This prevents
+the reader from seeing the incorrect head page.
+
+
+(first writer)
+
+               A        B    tail page
+               |        |        |
+               v        v        v
+    +---+    +---+    +---+    +---+
+<---|   |--->|   |--->|   |--->|   |-H->
+--->|   |<---|   |<---|   |<---|   |<---
+    +---+    +---+    +---+    +---+
+