Messages in this thread | | | Subject | Re: [PATCH net-next 03/12] ptr_ring: READ/WRITE_ONCE for __ptr_ring_empty | From | Jason Wang <> | Date | Fri, 26 Jan 2018 11:19:58 +0800 |
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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@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,
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
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