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SubjectRe: [patch] convert aio event reap to use atomic-op instead of spin_lock
On Wed, 11 Apr 2007 11:00:38 -0700
Zach Brown <zach.brown@oracle.com> wrote:

> > - /* Compensate for the ring buffer's head/tail overlap entry */
> > - nr_events += 2; /* 1 is required, 2 for good luck */
> > + /* round nr_event to next power of 2 */
> > + nr_events = roundup_pow_of_two(nr_events);
>
> Is that buggy? How will the code tell if head == tail means a full ring
> or an empty ring? The old code added that extra event to let it tell
> the ring was full before head == tail and it would think it's empty
> again, I think. I'm guessing roundup(nr_events + 1) is needed.

Ken uses the other (superior!) way of implementing ringbuffers: the head
and tail pointers (the naming of which AIO appears to have reversed) are
not constrained to the ringsize - they are simply allowed to wrap through
0xfffffff. Consequently:
ring full: (head-tail == size)
ring empty: head==tail
numer-of-items-in-ring: head-tail
add to ring: ring[head++]=item
remove from ring: item=ring[tail++]

(adjust the above for AIO naming assbackwardness)
(requires that size be a power of 2)

Many net drivers do it this way for their DMA rings.

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