linux-kernel - [PATCH net-next 02/12] ptr_ring: clean up documentation

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Message-ID: <1516923320-16959-3-git-send-email-mst@redhat.com>
Date:   Fri, 26 Jan 2018 01:36:28 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        David Miller <davem@...emloft.net>
Subject: [PATCH net-next 02/12] 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 <mst@...hat.com>
---
 include/linux/ptr_ring.h | 34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index 5ebcdd4..8594c7b 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);
-- 
MST