ptr_ring: keep consumer_head valid at all times

The comment near __ptr_ring_peek says:

 * 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.

but this was in fact never possible since consumer_head would sometimes
point outside the ring. Refactor the code so that it's always
pointing within a ring.

Fixes: c5ad119fb6c09 ("net: sched: pfifo_fast use skb_array")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 16 insertions, 9 deletions

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index 9ca1726ff963..5ebcdd40df99 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -248,22 +248,28 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r)
         /* Fundamentally, what we want to do is update consumer
          * index and zero out the entry so producer can reuse it.
          * Doing it naively at each consume would be as simple as:
-         *       r->queue[r->consumer++] = NULL;
-         *       if (unlikely(r->consumer >= r->size))
-         *               r->consumer = 0;
+         *       consumer = r->consumer;
+         *       r->queue[consumer++] = NULL;
+         *       if (unlikely(consumer >= r->size))
+         *               consumer = 0;
+         *       r->consumer = consumer;
          * but that is suboptimal when the ring is full as producer is writing
          * out new entries in the same cache line.  Defer these updates until a
          * batch of entries has been consumed.
          */
-        int head = r->consumer_head++;
+        /* Note: we must keep consumer_head valid at all times for __ptr_ring_empty
+         * to work correctly.
+         */
+        int consumer_head = r->consumer_head;
+        int head = consumer_head++;
 
         /* Once we have processed enough entries invalidate them in
          * the ring all at once so producer can reuse their space in the ring.
          * We also do this when we reach end of the ring - not mandatory
          * but helps keep the implementation simple.
          */
-        if (unlikely(r->consumer_head - r->consumer_tail >= r->batch ||
-                     r->consumer_head >= r->size)) {
+        if (unlikely(consumer_head - r->consumer_tail >= r->batch ||
+                     consumer_head >= r->size)) {
                 /* Zero out entries in the reverse order: this way we touch the
                  * cache line that producer might currently be reading the last;
                  * producer won't make progress and touch other cache lines
@@ -271,12 +277,13 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r)
                  */
                 while (likely(head >= r->consumer_tail))
                         r->queue[head--] = NULL;
-                r->consumer_tail = r->consumer_head;
+                r->consumer_tail = consumer_head;
         }
-        if (unlikely(r->consumer_head >= r->size)) {
-                r->consumer_head = 0;
+        if (unlikely(consumer_head >= r->size)) {
+                consumer_head = 0;
                 r->consumer_tail = 0;
         }
+        r->consumer_head = consumer_head;
 }
 
 static inline void *__ptr_ring_consume(struct ptr_ring *r)