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 <m...@redhat.com>
> > ---
> > 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
