From 8619d384eb5e9256a9d4ebe96c1832ac9b9049d6 Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Fri, 26 Jan 2018 01:36:28 +0200 Subject: ptr_ring: clean up documentation The only function safe to call without locks is __ptr_ring_empty. Move documentation about lockless use there to make sure people do not try to use __ptr_ring_peek outside locks. Signed-off-by: Michael S. Tsirkin Signed-off-by: David S. Miller --- include/linux/ptr_ring.h | 34 ++++++++++++++++++---------------- 1 file changed, 18 insertions(+), 16 deletions(-) (limited to 'include/linux/ptr_ring.h') diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h index 5ebcdd40df99..8594c7b37c15 100644 --- a/include/linux/ptr_ring.h +++ b/include/linux/ptr_ring.h @@ -169,21 +169,6 @@ static inline int ptr_ring_produce_bh(struct ptr_ring *r, void *ptr) return ret; } -/* Note: callers invoking this in a loop must use a compiler barrier, - * for example cpu_relax(). Callers must take consumer_lock - * 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) { if (likely(r->size)) @@ -191,7 +176,24 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r) return NULL; } -/* See __ptr_ring_peek above for locking rules. */ +/* + * Test ring empty status without taking any locks. + * + * NB: This is only safe to call if ring is never resized. + * + * However, 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 + * re-test __ptr_ring_empty and/or consume the ring enteries + * after the synchronization point. + * + * Note: callers invoking this in a loop must use a compiler barrier, + * for example cpu_relax(). + */ static inline bool __ptr_ring_empty(struct ptr_ring *r) { return !__ptr_ring_peek(r); -- cgit v1.2.3-59-g8ed1b