Re: [PATCH 3/5] ring-buffer: read page interface

Steven Rostedt
2008-12-02T22:25:21Z

On Tue, 2 Dec 2008, Andrew Morton wrote:

> On Tue, 02 Dec 2008 15:34:07 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > From: Steven Rostedt <srostedt@redhat.com>
> >
> > Impact: new API to ring buffer
> >
> > This patch adds a new interface into the ring buffer that allows a
> > page to be read from the ring buffer on a given CPU. For every page
> > read, one must also be given to allow for a "swap" of the pages.
> >
> > rpage = ring_buffer_alloc_read_page(buffer);
> > if (!rpage)
> > goto err;
> > ret = ring_buffer_read_page(buffer, &rpage, cpu, full);
> > if (!ret)
> > goto empty;
> > process_page(rpage);
> > ring_buffer_free_read_page(rpage);
> >
> > The caller of these functions must handle any waits that are
> > needed to wait for new data. The ring_buffer_read_page will simply
> > return 0 if there is no data, or if "full" is set and the writer
> > is still on the current page.
> >
> > ...
> >
> > +static inline void *
> > +__rb_data_page_index(struct buffer_data_page *page, unsigned index)
> > +{
> > + return page->data + index;
> > +}
>
> Many kernel developers will fall over stunned and confused when they
> see a variable called `page' which does not have type `struct page *'.
> This is a very strong kernel convention and this code tromps all over
> it :(
>
> How's about renaming all of these everywhere to `bpage' or whatever?

OK, I'll send a patch to Ingo to make this update.

>
> >
> > ...
> >
> > + if (!local_read(&cpu_buffer->reader_page->page->commit))
> >
>
> Rusty (I think it was Rusty) has convincingly argued that local_t is
> irredeemably busted. I forget the details?

Really?? I've been using local for quite a bit. Using atomic seems
overkill, since I only want to be able to protect against updates by
interrupts. Perhaps what needs to be done is to fix local?

-- Steve