Have the ring_buffer_iterator set a flag if events were dropped as it were
to go and peek at the next event. Have the trace file display this fact if
it happened with a "LOST EVENTS" message.

Link: http://lkml.kernel.org/r/20200317213417.045858900@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Now that the iterator can handle a concurrent writer, do not disable writing
to the ring buffer when there is an iterator present.

Link: http://lkml.kernel.org/r/20200317213416.759770696@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

When the ring buffer becomes writable for even when the trace file is read,
it must still not be resized. But since tracers can be activated while the
trace file is being read, the irqsoff tracer can modify the per CPU buffers,
and this can cause the reader of the trace file to update the wrong buffer's
resize disable bit, as the irqsoff tracer swaps out cpu buffers.

By making the resize disable per cpu_buffer, it makes the update follow the
per cpu_buffer even if it's swapped out with the snapshot buffer and keeps
the release of the trace file modifying the same data as the open did.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

As it is fine to perform several "peeks" of event data in the ring buffer
via the iterator before moving it forward, do not re-read the event, just
return what was read before. Otherwise, it can cause inconsistent results,
especially when testing multiple CPU buffers to interleave them.

Link: http://lkml.kernel.org/r/20200317213416.592032170@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

As the iterator will be reading a live buffer, and if the event being read
is on a page that a writer crosses, it will fail and try again, the
condition in rb_iter_peek() that only allows a retry to happen three times
is no longer valid. Allow rb_iter_peek() to retry more than three times
without killing the ring buffer, but only if rb_iter_head_event() had failed
at least once.

Link: http://lkml.kernel.org/r/20200317213416.452888193@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Have the ring_buffer_iter structure have a place to store an event, such
that it can not be overwritten by a writer, and load it in such a way via
rb_iter_head_event() that it will return NULL and reset the iter to the
start of the current page if a writer updated the page.

Link: http://lkml.kernel.org/r/20200317213416.306959216@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Have the ring_buffer_iter structure contain a page_stamp, such that it can
be used to see if the writer entered the page the iterator is on. When going
to a new page, the iterator will record the time stamp of that page. When
reading events, it can copy the event to an internal buffer on the iterator
(to be implemented later), then check the page's time stamp with its own to
see if the writer entered the page. If so, it will need to try to read the
event again.

Link: http://lkml.kernel.org/r/20200317213416.163549674@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

When the ring buffer was first created, the iterator followed the normal
producer/consumer operations where it had both a peek() operation, that just
returned the event at the current location, and a read(), that would return
the event at the current location and also increment the iterator such that
the next peek() or read() will return the next event.

The only use of the ring_buffer_read() is currently to move the iterator to
the next location and nothing now actually reads the event it returns.
Rename this function to its actual use case to ring_buffer_iter_advance(),
which also adds the "iter" part to the name, which is more meaningful. As
the timestamp returned by ring_buffer_read() was never used, there's no
reason that this new version should bother having returning it. It will also
become a void function.

Link: http://lkml.kernel.org/r/20200317213416.018928618@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

It was complained about that when the trace file is read, that the tracing
is disabled, as the iterator expects writing to the buffer it reads is not
updated. Several steps are needed to make the iterator handle a writer,
by testing if things have changed as it reads.

This step is to make ring_buffer_empty() expect the buffer to be changing.
Note if the current location of the iterator is overwritten, then it will
return false as new data is being added. Note, that this means that data
will be skipped.

Link: http://lkml.kernel.org/r/20200317213415.870741809@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

This macro isn't used since commit d325c402964e ("ring-buffer: Remove
unused function ring_buffer_page_len()"), so better to remove it.

Link: http://lkml.kernel.org/r/1579586080-45300-1-git-send-email-alex.shi@linux.alibaba.com

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>