On Thu, 2012-06-28 at 19:16 -0400, Steven Rostedt wrote: > From: Steven Rostedt <[email protected]> > > The ring buffer reader page is used to swap a page from the writable > ring buffer. If the writer happens to be on that page, it ends up on the > reader page, but will simply move off of it, back into the writable ring > buffer as writes are added. > > The time stamp passed back to the readers is stored in the cpu_buffer per > CPU descriptor. This stamp is updated when a swap of the reader page takes > place, and it reads the current stamp from the page taken from the writable > ring buffer. Everytime a writer goes to a new page, it updates the time stamp > of that page. > > The problem happens if a reader reads a page from an empty per CPU ring > buffer. > If the buffer is empty, the swap still takes place, placing the writer at the > start of the reader page. If at a later time, a write happens, it updates the > page's time stamp and continues. But the problem is that the read_stamp does > not get updated, because the page was already swapped. > > The solution to this was to not swap the page if the ring buffer happens to > be empty. This also removes the side effect that the writes on the reader > page will not get updated because the writer never gets back on the reader > page without a swap. That is, if a read happens on an empty buffer, but then > no reads happen for a while. If a swap took place, and the writer were to > start > writing a lot of data (function tracer), it will start overflowing the ring > buffer > and overwrite the older data. But because the writer never goes back onto the > reader page, the data left on the reader page never gets overwritten. This > causes the reader to see really old data, followed by a jump to newer data. > > Link: > http://lkml.kernel.org/r/[email protected] > Google-Bug-Id: 6410455 > Reported-by: David Sharp <[email protected]> > tested-by: David Sharp <[email protected]> > Signed-off-by: Steven Rostedt <[email protected]>
I'm starting to consider that this patch should be in stable. Ingo, should I push this to urgent? -- Steve > --- > kernel/trace/ring_buffer.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 1d0f6a8..82a3e0c 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -3239,6 +3239,10 @@ rb_get_reader_page(struct ring_buffer_per_cpu > *cpu_buffer) > if (cpu_buffer->commit_page == cpu_buffer->reader_page) > goto out; > > + /* Don't bother swapping if the ring buffer is empty */ > + if (rb_num_of_entries(cpu_buffer) == 0) > + goto out; > + > /* > * Reset the reader page to size zero. > */ -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
