* Steven Rostedt <[email protected]> wrote:
> 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?
Yeah, probably makes sense to do so, especially as it's rather
small.
Thanks,
Ingo
--
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
