[RFC PATCH 04/10] pipe: Use head and tail pointers for the ring, not cursor and length [ver #2]

[Ilya Dryomov]

On Wed, Oct 30, 2019 at 9:35 PM Rasmus Villemoes
<linux at rasmusvillemoes.dk> wrote:
>
> On 30/10/2019 17.19, Ilya Dryomov wrote:
> > On Thu, Oct 24, 2019 at 11:49 AM David Howells <dhowells at redhat.com> wrote:
> >>  /*
> >> - * We use a start+len construction, which provides full use of the
> >> - * allocated memory.
> >> - * -- Florian Coosmann (FGC)
> >> - *
> >> + * We use head and tail indices that aren't masked off, except at the point of
> >> + * dereference, but rather they're allowed to wrap naturally.  This means there
> >> + * isn't a dead spot in the buffer, provided the ring size < INT_MAX.
> >> + * -- David Howells 2019-09-23.
> >
> > Hi David,
> >
> > Is "ring size < INT_MAX" constraint correct?
>
> No. As long as one always uses a[idx % size] to access the array, the
> only requirement is that size is representable in an unsigned int. Then
> because one also wants to do the % using simple bitmasking, that further
> restricts one to sizes that are a power of 2, so the end result is that
> the max size is 2^31 (aka INT_MAX+1).

I think the fact that indices are free running and wrap at a power of
two already restricts you to sizes the are a power of two, independent
of how you do masking.  If you switch to a[idx % size], size still has
to be a power of two for things to work when idx wraps.  Consider:

  size = 6
  head = tail = 4294967292, empty buffer

  push  4294967292 % 6 = 0
  push  4294967293 % 6 = 1
  push  4294967294 % 6 = 2
  push  4294967295 % 6 = 3
  push           0 % 6 = 0  <-- expected 4, overwrote a[0]

>
> > I've never had to implement this free running indices scheme, but
> > the way I've always visualized it is that the top bit of the index is
> > used as a lap (as in a race) indicator, leaving 31 bits to work with
> > (in case of unsigned ints).  Should that be
> >
> >   ring size <= 2^31
> >
> > or more precisely
> >
> >   ring size is a power of two <= 2^31
>
> Exactly. But it's kind of moot since the ring size would never be
> allowed to grow anywhere near that.

Thanks for confirming.  Even if it's kind of moot, I think it should be
corrected to avoid confusion.

                Ilya