Messages in this thread | | | Date | Fri, 6 Jul 2012 11:11:53 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 4/4] ring-buffer: Fix uninitialized read_stamp |
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* Steven Rostedt <rostedt@goodmis.org> wrote:
> On Thu, 2012-06-28 at 19:16 -0400, Steven Rostedt wrote: > > From: Steven Rostedt <srostedt@redhat.com> > > > > 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/1340060577-9112-1-git-send-email-dhsharp@google.com > > Google-Bug-Id: 6410455 > > Reported-by: David Sharp <dhsharp@google.com> > > tested-by: David Sharp <dhsharp@google.com> > > Signed-off-by: Steven Rostedt <rostedt@goodmis.org> > > 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
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