Re: [PATCH net-next 03/12] ptr_ring: READ/WRITE_ONCE for __ptr_ring_empty

[Jason Wang]

On 2018å01æ26æ 10:44, Michael S. Tsirkin wrote:
> On Fri, Jan 26, 2018 at 10:37:58AM +0800, Jason Wang wrote:
> > On 2018å01æ26æ 07:36, Michael S. Tsirkin wrote:
> > > Lockless __ptr_ring_empty requires that consumer head is read and
> > > written at once, atomically. Annotate accordingly to make sure compiler
> > > does it correctly.  Switch locked callers to __ptr_ring_peek which does
> > > not support the lockless operation.
> > >
> > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > ---
> > >    include/linux/ptr_ring.h | 11 ++++++++---
> > >    1 file changed, 8 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> > > index 8594c7b..9a72d8f 100644
> > > --- a/include/linux/ptr_ring.h
> > > +++ b/include/linux/ptr_ring.h
> > > @@ -196,7 +196,9 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r)
> > >     */
> > >    static inline bool __ptr_ring_empty(struct ptr_ring *r)
> > >    {
> > > -	return !__ptr_ring_peek(r);
> > > +	if (likely(r->size))
> > > +		return !r->queue[READ_ONCE(r->consumer_head)];
> > > +	return true;
> > >    }
> > So after patch 8, __ptr_ring_peek() did:
> >
> > static inline void *__ptr_ring_peek(struct ptr_ring *r)
> > {
> >  ÂÂÂ if (likely(r->size))
> >  ÂÂÂ ÂÂÂ return READ_ONCE(r->queue[r->consumer_head]);
> >  ÂÂÂ return NULL;
> > }
> >
> > Looks like a duplication.
> >
> > Thanks
> Nope - they are different.
>
> The reason is that __ptr_ring_peek does not need to read the consumer_head once
> since callers have a lock,

I get this.

>   and __ptr_ring_empty does not need to read
> the queue once since it merely compares it to 0.

Do this still work if it was called inside a loop?

Thanks