[PATCH 00/12 v2] ring-buffer/tracing: Remove disabling of ring buffer while reading trace file

[Steven Rostedt]

When the ring buffer was first written for ftrace, there was two
human readable files to read it. One was a standard "producer/consumer"
file (trace_pipe), which would consume data from the ring buffer as
it read it, and the other was a "static iterator" that would not
consume the events, such that the file could be read multiple times
and return the same output each time.

The "static iterator" was never meant to be read while there was an
active writer to the ring buffer. If writing was enabled, then it
would disable the writer when the trace file was opened.

There has been some complaints about this by the BPF folks, that did
not realize this little bit of information and it was requested that
the "trace" file does not stop the writing to the ring buffer.

This patch series attempts to satisfy that request, by creating a
temporary buffer in each of the per cpu iterators to place the
read event into, such that it can be passed to users without worrying
about a writer to corrupt the event while it was being written out.
It also uses the fact that the ring buffer is broken up into pages,
where each page has its own timestamp that gets updated when a
writer crosses over to it. By copying it to the temp buffer, and
doing a "before and after" test of the time stamp with memory barriers,
can allow the events to be saved.

Changes since v1:

 - Added fix to selftest first, where these changes wont break it

 - Changed comment in trace_find_next_entry() to better explain what
   it was doing, as pointed out by Masami Hiramatsu.

 - Allocated the iterator temp buffer when the iterator is created,
   as Masami pointed out, it would be better than allocating it each
   time it was used. It is initiated as 128 bytes as most trace events
   are less than that, but will be expanded if needed. Note that
   function is only used when latency measurements are needed (seeing
   two events at once).

Steven Rostedt (VMware) (12):
      selftest/ftrace: Fix function trigger test to handle trace not disabling the tracer
      tracing: Save off entry when peeking at next entry
      ring-buffer: Have ring_buffer_empty() not depend on tracing stopped
      ring-buffer: Rename ring_buffer_read() to read_buffer_iter_advance()
      ring-buffer: Add page_stamp to iterator for synchronization
      ring-buffer: Have rb_iter_head_event() handle concurrent writer
      ring-buffer: Do not die if rb_iter_peek() fails more than thrice
      ring-buffer: Optimize rb_iter_head_event()
      ring-buffer: Do not disable recording when there is an iterator
      tracing: Do not disable tracing when reading the trace file
      ring-buffer/tracing: Have iterator acknowledge dropped events
      tracing: Have the document reflect that the trace file keeps tracing enabled

----
 Documentation/trace/ftrace.rst                     |  13 +-
 include/linux/ring_buffer.h                        |   4 +-
 include/linux/trace_events.h                       |   2 +
 kernel/trace/ring_buffer.c                         | 196 +++++++++++++++------
 kernel/trace/trace.c                               |  68 +++++--
 kernel/trace/trace_functions_graph.c               |   2 +-
 kernel/trace/trace_output.c                        |  15 +-
 .../test.d/ftrace/func_traceonoff_triggers.tc      |   2 +-
 8 files changed, 211 insertions(+), 91 deletions(-)