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

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Date:   Fri, 26 Jan 2018 15:44:15 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next 03/12] ptr_ring: READ/WRITE_ONCE for
 __ptr_ring_empty

On Fri, Jan 26, 2018 at 11:19:58AM +0800, Jason Wang wrote:
> 
> 
> On 2018年01月26日 10:44, Michael S. Tsirkin wrote:
> > 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 <mst@...hat.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,
> 
> 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

Sure because compiler does not know head didn't change.

-- 
MST