Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 31 Oct 2012 10:11:06 +0200 Janne Kulmala <janne.t.kulmala@tut.fi> wrote:

> On 10/31/2012 08:52 AM, Andrew Morton wrote:
> > On Wed, 31 Oct 2012 07:30:33 +0100 Stefani Seibold <stefani@seibold.net> wrote:
> >
> >>> Yes, and I guess the same to give them a 64-element one.
> >>>
> >>>>
> >>>> If there's absolutely no prospect that the kfifo code will ever support
> >>>> 100-byte fifos then I guess we should rework the API so that the caller
> >>>> has to pass in log2 of the size, not the size itself.  That way there
> >>>> will be no surprises and no mistakes.
> >>>>
> >>>> That being said, the power-of-2 limitation isn't at all intrinsic to a
> >>>> fifo, so we shouldn't do this.  Ideally, we'd change the kfifo
> >>>> implementation so it does what the caller asked it to do!
> >>>
> >>> I'm fine with removing the power-of-2 limitation. Stefani, what's your
> >>> comment on that?
> >>>
> >>
> >> You can't remove the power-of-2-limitation, since this would result in a
> >> performance decrease (bit wise and vs. modulo operation).
> >
> > Probably an insignificant change in performance.
> >
> > It could be made much smaller by just never doing the modulus operation
> > - instead do
> >
> > 	if (++index == max)
> > 		index = 0;
> >
> 
> This can not be done, since the index manipulation kfifo does not use locks.

Oh come on.  Look:

                        __kfifo->out++; \

and look:

 * Note that with only one concurrent reader and one concurrent
 * writer, you don't need extra locking to use these macro.