From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Fri, 26 Jan 2018 01:36:29 +0200
Subject: ptr_ring: READ/WRITE_ONCE for __ptr_ring_empty
Patch-mainline: v4.16-rc1
Git-commit: a259df36d1fbf25f0e4f649fdb84e4527e5640ed
References: bsc#1109837

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>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 include/linux/ptr_ring.h |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -196,7 +196,9 @@ static inline void *__ptr_ring_peek(stru
  */
 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;
 }
 
 static inline bool ptr_ring_empty(struct ptr_ring *r)
@@ -285,7 +287,8 @@ static inline void __ptr_ring_discard_on
 		consumer_head = 0;
 		r->consumer_tail = 0;
 	}
-	r->consumer_head = consumer_head;
+	/* matching READ_ONCE in __ptr_ring_empty for lockless tests */
+	WRITE_ONCE(r->consumer_head, consumer_head);
 }
 
 static inline void *__ptr_ring_consume(struct ptr_ring *r)
@@ -536,7 +539,9 @@ static inline void ptr_ring_unconsume(st
 			goto done;
 		}
 		r->queue[head] = batch[--n];
-		r->consumer_tail = r->consumer_head = head;
+		r->consumer_tail = head;
+		/* matching READ_ONCE in __ptr_ring_empty for lockless tests */
+		WRITE_ONCE(r->consumer_head, head);
 	}
 
 done: