ck_ring: add a ck_ring_seek_* family of functions.

These functions will forward consumer counter without paying cost of
copying data out of ring buffer.

include/ck_ring.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2009-2015 Samy Al Bahra.
+ * Copyright 2009-2019 Samy Al Bahra.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -130,6 +130,21 @@ _ck_ring_enqueue_sp_size(struct ck_ring *ring,
 	return r;
 }
 
+CK_CC_FORCE_INLINE static bool
+_ck_ring_seek_sc(struct ck_ring *ring)
+{
+	unsigned int consumer, producer;
+
+	consumer = ring->c_head;
+	producer = ck_pr_load_uint(&ring->p_tail);
+
+	if (CK_CC_UNLIKELY(consumer == producer))
+		return false;
+
+	ck_pr_store_uint(&ring->c_head, consumer + 1);
+	return true;
+}
+
 CK_CC_FORCE_INLINE static bool
 _ck_ring_dequeue_sc(struct ck_ring *ring,
     const void *CK_CC_RESTRICT buffer,
@@ -264,6 +279,46 @@ _ck_ring_enqueue_mp_size(struct ck_ring *ring,
 	return r;
 }
 
+CK_CC_FORCE_INLINE static bool
+_ck_ring_tryseek_mc(struct ck_ring *ring)
+{
+	unsigned int consumer, producer;
+
+	consumer = ck_pr_load_uint(&ring->c_head);
+	ck_pr_fence_load();
+	producer = ck_pr_load_uint(&ring->p_tail);
+
+	if (CK_CC_UNLIKELY(consumer == producer))
+		return false;
+
+	return ck_pr_cas_uint(&ring->c_head, consumer, consumer + 1);
+}
+
+CK_CC_FORCE_INLINE static bool
+_ck_ring_seek_mc(struct ck_ring *ring)
+{
+	unsigned int consumer, producer;
+
+	consumer = ck_pr_load_uint(&ring->c_head);
+
+	do {
+		/*
+		 * Producer counter must represent state relative to
+		 * our latest consumer snapshot.
+		 */
+		ck_pr_fence_load();
+		producer = ck_pr_load_uint(&ring->p_tail);
+
+		if (CK_CC_UNLIKELY(consumer == producer))
+			return false;
+	} while (ck_pr_cas_uint_value(&ring->c_head,
+				      consumer,
+				      consumer + 1,
+				      &consumer) == false);
+
+	return true;
+}
+
 CK_CC_FORCE_INLINE static bool
 _ck_ring_trydequeue_mc(struct ck_ring *ring,
     const void *buffer,
@@ -354,6 +409,13 @@ ck_ring_enqueue_spsc(struct ck_ring *ring,
 	    &entry, sizeof(entry), NULL);
 }
 
+CK_CC_INLINE static bool
+ck_ring_seek_spsc(struct ck_ring *ring)
+{
+
+	return _ck_ring_seek_sc(ring);
+}
+
 CK_CC_INLINE static bool
 ck_ring_dequeue_spsc(struct ck_ring *ring,
     const struct ck_ring_buffer *buffer,
@@ -410,6 +472,20 @@ ck_ring_dequeue_mpmc(struct ck_ring *ring,
 	    sizeof(void *));
 }
 
+CK_CC_INLINE static bool
+ck_ring_tryseek_mpmc(struct ck_ring *ring)
+{
+
+	return _ck_ring_tryseek_mc(ring);
+}
+
+CK_CC_INLINE static bool
+ck_ring_seek_mpmc(struct ck_ring *ring)
+{
+
+	return _ck_ring_seek_mc(ring);
+}
+
 /*
  * The ck_ring_*_spmc namespace is the public interface for interacting with a
  * ring buffer containing pointers. Correctness is provided for any number of
@@ -454,6 +530,20 @@ ck_ring_dequeue_spmc(struct ck_ring *ring,
 	return _ck_ring_dequeue_mc(ring, buffer, (void **)data, sizeof(void *));
 }
 
+CK_CC_INLINE static bool
+ck_ring_tryseek_spmc(struct ck_ring *ring)
+{
+
+	return _ck_ring_tryseek_mc(ring);
+}
+
+CK_CC_INLINE static bool
+ck_ring_seek_spmc(struct ck_ring *ring)
+{
+
+	return _ck_ring_seek_mc(ring);
+}
+
 /*
  * The ck_ring_*_mpsc namespace is the public interface for interacting with a
  * ring buffer containing pointers. Correctness is provided for any number of
@@ -480,6 +570,13 @@ ck_ring_enqueue_mpsc_size(struct ck_ring *ring,
 	    sizeof(entry), size);
 }
 
+CK_CC_INLINE static bool
+ck_ring_seek_mpsc(struct ck_ring *ring)
+{
+
+	return _ck_ring_seek_sc(ring);
+}
+
 CK_CC_INLINE static bool
 ck_ring_dequeue_mpsc(struct ck_ring *ring,
     const struct ck_ring_buffer *buffer,

regressions/ck_ring/validate/ck_ring_mpmc.c

@@ -102,11 +102,13 @@ test_mpmc(void *c)
 				if (ck_ring_dequeue_mpmc(&ring_mw, buffer, &o) == false)
 					o = NULL;
 			} else {
+				ck_ring_seek_mpmc(&ring_mw);
 				if (ck_ring_trydequeue_mpmc(&ring_mw, buffer, &o) == false)
 					o = NULL;
 			}
 
 			if (o == NULL) {
+				for (size_t z = 0; z < 2; z++) {
 					o = malloc(sizeof(*o));
 					if (o == NULL)
 						continue;
@@ -122,6 +124,7 @@ test_mpmc(void *c)
 					} else {
 						enqueue++;
 					}
+				}
 
 				continue;
 			}

regressions/ck_ring/validate/ck_ring_spmc.c

@@ -327,7 +327,7 @@ main(int argc, char *argv[])
 			}
 
 			if ((int)s >= (size * ITERATIONS * (nthr - 1))) {
-				ck_error("MPMC: Unexpected size of %u\n", s);
+				ck_error("SPMC: Unexpected size of %u\n", s);
 			}
 		}
 	}