linux-kernel - Re: kfifo has temporarily invalid in pointer?

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Date:	Mon, 15 Mar 2010 13:17:49 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...shcourse.ca>
To:	Daniel Baluta <daniel.baluta@...il.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kfifo has temporarily invalid in pointer?

On Mon, 15 Mar 2010, Daniel Baluta wrote:

> Hi Robert,
>
> On Mon, Mar 15, 2010 at 4:58 PM, Robert P. J. Day <rpjday@...shcourse.ca> wrote:
> >
> >  (i am not trying to be annoyingly obsessive about the kernel kfifo,
> > i am merely succeeding.)
> :P
> >  what appears to be a bit of an oddity WRT kfifo:  since a kfifo is
> > defined with a fixed buffer size, it obviously enqueues and dequeues
> > in a circular fashion.  so, the code to add some data to a kfifo (from
> > kernel/kfifo.c):
> >
> > =====
> > unsigned int kfifo_in(struct kfifo *fifo, const void *from,
> >                                unsigned int len)
> > {
> >        len = min(kfifo_avail(fifo), len);
> >
> >        __kfifo_in_data(fifo, from, len, 0);
> >        __kfifo_add_in(fifo, len);
> >        return len;
> > }
> > =====
> >
> >  fair enough -- that first routine adds the data itself, while the
> > second one correspondingly bumps up the pointer, which could
> > conceivably wrap around to follow the data, correct?  but from
> > include/linux.kfifo.h:len = min(kfifo_avail(fifo), len);
>
> Wrong :). If you notice len is truncated using:
> len = min(kfifo_avail(fifo), len);

  kfifo_avail() is defined as returning the number of available bytes
left in the buffer ready to accept incoming data, even if that
incorporates wraparound.  that is not relevant to the point i was
making.

rday
--

========================================================================
Robert P. J. Day                               Waterloo, Ontario, CANADA

            Linux Consulting, Training and Kernel Pedantry.

Web page:                                          http://crashcourse.ca
Twitter:                                       http://twitter.com/rpjday
========================================================================