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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 26 Jan 2018 04:44:12 +0200

On Fri, Jan 26, 2018 at 10:37:58AM +0800, Jason Wang wrote:
> 
> 
> On 2018年01月26日 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 <[email-masked]>
> > ---
> >   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)
> {
>     if (likely(r->size))
>         return READ_ONCE(r->queue[r->consumer_head]);
>     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, and __ptr_ring_empty does not need to read
the queue once since it merely compares it to 0.

-- 
MST