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

From: Andrew Morton
Date: Tue Dec 02 2008 - 16:39:01 EST

On Tue, 02 Dec 2008 15:34:07 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> From: Steven Rostedt <srostedt@xxxxxxxxxx>
> 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?

> ...
> + 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?

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