ck_fifo: MPMC variant will return "garbage" pointer which user can re-use.

include/ck_fifo.h

@@ -155,8 +155,6 @@ struct ck_fifo_mpmc {
 	struct ck_fifo_mpmc_pointer head;
 	char pad[CK_MD_CACHELINE - sizeof(struct ck_fifo_mpmc_pointer)];
 	struct ck_fifo_mpmc_pointer tail;
-	struct ck_fifo_mpmc_entry *head_snapshot;
-	struct ck_fifo_mpmc_pointer garbage;
 };
 typedef struct ck_fifo_mpmc ck_fifo_mpmc_t;
 
@@ -170,8 +168,6 @@ ck_fifo_mpmc_init(struct ck_fifo_mpmc *fifo, struct ck_fifo_mpmc_entry *stub)
 	ck_pr_store_ptr(&fifo->tail.generation, 0);
 	ck_pr_store_ptr(&stub->next.pointer, NULL);
 	ck_pr_store_ptr(&stub->next.generation, 0);
-	ck_pr_store_ptr(&fifo->head_snapshot, stub);
-	ck_pr_store_ptr(&fifo->garbage, stub);
 	return;
 }
 
@@ -233,7 +229,7 @@ ck_fifo_mpmc_enqueue(struct ck_fifo_mpmc *fifo, struct ck_fifo_mpmc_entry *entry
 }
 
 CK_CC_INLINE static bool
-ck_fifo_mpmc_dequeue(struct ck_fifo_mpmc *fifo, void *value)
+ck_fifo_mpmc_dequeue(struct ck_fifo_mpmc *fifo, void *value, ck_fifo_mpmc_entry_t **garbage)
 {
 	struct ck_fifo_mpmc_pointer head, tail, next, update;
 	ck_backoff_t backoff = CK_BACKOFF_INITIALIZER;
@@ -274,6 +270,7 @@ ck_fifo_mpmc_dequeue(struct ck_fifo_mpmc *fifo, void *value)
 		}
 	}
 
+	*garbage = head.pointer;
 	return (true);
 }
 

regressions/ck_fifo/benchmark/latency.c

@@ -59,6 +59,7 @@ main(void)
 	ck_fifo_mpmc_t mpmc_fifo;
 	ck_fifo_mpmc_entry_t mpmc_entry[ENTRIES];
 	ck_fifo_mpmc_entry_t mpmc_stub;
+	ck_fifo_mpmc_entry_t *garbage;
 #endif
 
 #ifdef CK_F_FIFO_SPSC
@@ -145,7 +146,7 @@ main(void)
 
 		s = rdtsc();
 		for (j = 0; j < ENTRIES; j++)
-			ck_fifo_mpmc_dequeue(&mpmc_fifo, &r);
+			ck_fifo_mpmc_dequeue(&mpmc_fifo, &r, &garbage);
 		e = rdtsc();
 		a += e - s;
 	}

regressions/ck_fifo/validate/ck_fifo_mpmc.c

@@ -64,7 +64,7 @@ test(void *c)
 #ifdef CK_F_FIFO_MPMC
 	struct context *context = c;
 	struct entry *entry;
-	ck_fifo_mpmc_entry_t *fifo_entry;
+	ck_fifo_mpmc_entry_t *fifo_entry, *garbage;
 	int i, j;
 
         if (aff_iterate(&a)) {
@@ -81,7 +81,7 @@ test(void *c)
 			entry = malloc(sizeof(struct entry));
 			entry->tid = context->tid;
 			ck_fifo_mpmc_enqueue(&fifo, fifo_entry, entry);
-			if (ck_fifo_mpmc_dequeue(&fifo, &entry) == false) {
+			if (ck_fifo_mpmc_dequeue(&fifo, &entry, &garbage) == false) {
 				fprintf(stderr, "ERROR [%u] Queue should never be empty.\n", context->tid);
 				exit(EXIT_FAILURE);
 			}