From 66940f35d5a81d5969bb5543171c70a434fc5110 Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Wed, 10 Jan 2018 16:03:05 +0200 Subject: ptr_ring: document usage around __ptr_ring_peek This explains why is the net usage of __ptr_ring_peek actually ok without locks. Signed-off-by: Michael S. Tsirkin Acked-by: John Fastabend Signed-off-by: David S. Miller --- include/linux/ptr_ring.h | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) (limited to 'include/linux/ptr_ring.h') diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h index 6866df4f31b5..d72b2e7dd500 100644 --- a/include/linux/ptr_ring.h +++ b/include/linux/ptr_ring.h @@ -174,6 +174,15 @@ static inline int ptr_ring_produce_bh(struct ptr_ring *r, void *ptr) * if they dereference the pointer - see e.g. PTR_RING_PEEK_CALL. * If ring is never resized, and if the pointer is merely * tested, there's no need to take the lock - see e.g. __ptr_ring_empty. + * However, if called outside the lock, and if some other CPU + * consumes ring entries at the same time, the value returned + * is not guaranteed to be correct. + * In this case - to avoid incorrectly detecting the ring + * as empty - the CPU consuming the ring entries is responsible + * for either consuming all ring entries until the ring is empty, + * or synchronizing with some other CPU and causing it to + * execute __ptr_ring_peek and/or consume the ring enteries + * after the synchronization point. */ static inline void *__ptr_ring_peek(struct ptr_ring *r) { @@ -182,10 +191,7 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r) return NULL; } -/* Note: callers invoking this in a loop must use a compiler barrier, - * for example cpu_relax(). Callers must take consumer_lock - * if the ring is ever resized - see e.g. ptr_ring_empty. - */ +/* See __ptr_ring_peek above for locking rules. */ static inline bool __ptr_ring_empty(struct ptr_ring *r) { return !__ptr_ring_peek(r); -- cgit v1.2.3-59-g8ed1b