ck_backoff: Saturate geometric back-off.

A simple regression test has been added to verify this behavior.

include/ck_backoff.h

@@ -30,10 +30,10 @@
 #include <ck_cc.h>
 
 #ifndef CK_BACKOFF_CEILING
-#define CK_BACKOFF_CEILING ((1 << 21) - 1)
+#define CK_BACKOFF_CEILING ((1 << 20) - 1)
 #endif
 
-#define CK_BACKOFF_INITIALIZER ((1 << 9) - 1)
+#define CK_BACKOFF_INITIALIZER (1 << 9)
 
 typedef volatile unsigned int ck_backoff_t;
 
@@ -71,10 +71,7 @@ ck_backoff_gb(volatile unsigned int *c)
 
 	for (i = 0; i < ceiling; i++);
 
-	ceiling <<= 1;
-	ceiling &= CK_BACKOFF_CEILING;
-
-	*c = ceiling;
+	*c = ceiling <<= ceiling < CK_BACKOFF_CEILING;
 	return;
 }
 

regressions/Makefile

@@ -2,6 +2,7 @@
 
 all:
 	$(MAKE) -C ./ck_bitmap/validate all
+	$(MAKE) -C ./ck_backoff/validate all
 	$(MAKE) -C ./ck_queue/validate all
 	$(MAKE) -C ./ck_brlock/validate all
 	$(MAKE) -C ./ck_ht/validate all
@@ -30,6 +31,7 @@ all:
 	$(MAKE) -C ./ck_bag/validate all
 
 clean:
+	$(MAKE) -C ./ck_backoff/validate clean
 	$(MAKE) -C ./ck_bitmap/validate clean
 	$(MAKE) -C ./ck_queue/validate clean
 	$(MAKE) -C ./ck_brlock/validate clean

regressions/ck_backoff/validate/Makefile

@@ -0,0 +1,11 @@
+.PHONY: clean
+
+all: validate
+
+validate: validate.c ../../../include/ck_backoff.h
+	$(CC) $(CFLAGS) -o validate validate.c
+
+clean:
+	rm -rf validate *.dSYM
+
+include ../../../build/regressions.build

regressions/ck_backoff/validate/validate.c

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2011-2012 Samy Al Bahra.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <ck_backoff.h>
+#include "../../common.h"
+
+int
+main(void)
+{
+	ck_backoff_t backoff = CK_BACKOFF_INITIALIZER;
+	const ck_backoff_t ceiling = CK_BACKOFF_CEILING + 1;
+	unsigned int i = 0;
+
+	fprintf(stderr, "Ceiling is: %u (%#x)\n", CK_BACKOFF_CEILING, CK_BACKOFF_CEILING);
+
+	for (;;) {
+		ck_backoff_t previous = backoff;
+		ck_backoff_gb(&backoff);
+
+		if (previous == ceiling) {
+			if (backoff != ceiling) {
+				fprintf(stderr, "[C] GB: expected %u, got %u\n", ceiling, backoff);
+				exit(EXIT_FAILURE);
+			}
+
+			if (i++ >= 1)
+				break;
+		} else if (previous != backoff >> 1) {
+			fprintf(stderr, "[N] GB: expected %u (%u), got %u\n", previous << 1, previous, backoff);
+			exit(EXIT_FAILURE);
+		}
+	}
+
+	backoff = CK_BACKOFF_INITIALIZER;
+	for (;;) {
+		ck_backoff_t previous = backoff;
+		ck_backoff_eb(&backoff);
+
+		if (backoff = previous) {
+			if (i++ >= 3)
+				break;
+		} else if (previous * previous >= backoff) {
+			fprintf(stderr, "[N] EB: expected %u (%u), got %u\n", previous * previous, previous, backoff);
+			exit(EXIT_FAILURE);
+		}
+	}
+
+	return 0;
+}
+