Use proper padding instead of using alignas()

As it was pointed out, the alignas() can't be used on objects larger
than `max_align_t` otherwise the compiler might miscompile the code to
use auto-vectorization on unaligned memory.

As we were only using alignas() as a way to prevent false memory
sharing, we can use manual padding in the affected structures.

configure.ac

@@ -388,7 +388,7 @@ AC_COMPILE_IFELSE(
   ],
   [AC_MSG_FAILURE([stdatomic.h header found, but compilation failed, please fix your toolchain.])])
 
-AC_CHECK_HEADERS([stdalign.h stdnoreturn.h],
+AC_CHECK_HEADERS([stdnoreturn.h],
 		 [],
 		 [AC_MSG_ERROR([C11 standard headers not found, update your toolchain.])])
 

lib/isc/Makefile.am

@@ -4,7 +4,6 @@ lib_LTLIBRARIES = libisc.la
 
 libisc_ladir = $(includedir)/isc
 libisc_la_HEADERS =			\
-	include/isc/align.h		\
 	include/isc/ascii.h		\
 	include/isc/assertions.h	\
 	include/isc/async.h		\

lib/isc/include/isc/align.h

@@ -1,20 +0,0 @@
-/*
- * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
- *
- * SPDX-License-Identifier: MPL-2.0
- *
- * This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, you can obtain one at https://mozilla.org/MPL/2.0/.
- *
- * See the COPYRIGHT file distributed with this work for additional
- * information regarding copyright ownership.
- */
-
-#pragma once
-
-#ifdef HAVE_STDALIGN_H
-#include <stdalign.h>
-#else /* ifdef HAVE_STDALIGN_H */
-#define alignas(x) __attribute__((__aligned__(x)))
-#endif /* ifdef HAVE_STDALIGN_H */

lib/isc/include/isc/quota.h

@@ -30,7 +30,6 @@
  *** Imports.
  ***/
 
-#include <isc/align.h>
 #include <isc/atomic.h>
 #include <isc/job.h>
 #include <isc/lang.h>
@@ -57,14 +56,19 @@ ISC_LANG_BEGINDECLS
  * synchronization between multiple threads (see urcu/wfcqueue.h for
  * detailed description).
  */
+STATIC_ASSERT(ISC_OS_CACHELINE_SIZE >= sizeof(struct __cds_wfcq_head),
+	      "ISC_OS_CACHELINE_SIZE smaller than "
+	      "sizeof(struct __cds_wfcq_head)");
 struct isc_quota {
 	int		     magic;
 	atomic_uint_fast32_t max;
 	atomic_uint_fast32_t used;
 	atomic_uint_fast32_t soft;
 	struct {
-		alignas(ISC_OS_CACHELINE_SIZE) struct cds_wfcq_head head;
+		struct cds_wfcq_head head;
-		alignas(ISC_OS_CACHELINE_SIZE) struct cds_wfcq_tail tail;
+		uint8_t		     __padding[ISC_OS_CACHELINE_SIZE -
+				       sizeof(struct __cds_wfcq_head)];
+		struct cds_wfcq_tail tail;
 	} jobs;
 	ISC_LINK(isc_quota_t) link;
 };

lib/isc/include/isc/rwlock.h

@@ -161,15 +161,23 @@ typedef pthread_rwlock_t isc__rwlock_t;
 
 #else /* USE_PTHREAD_RWLOCK */
 
-#include <isc/align.h>
 #include <isc/atomic.h>
 #include <isc/os.h>
 
+STATIC_ASSERT(ISC_OS_CACHELINE_SIZE >= sizeof(atomic_uint_fast32_t),
+	      "ISC_OS_CACHELINE_SIZE smaller than "
+	      "sizeof(atomic_uint_fast32_t)");
+STATIC_ASSERT(ISC_OS_CACHELINE_SIZE >= sizeof(atomic_int_fast32_t),
+	      "ISC_OS_CACHELINE_SIZE smaller than sizeof(atomic_int_fast32_t)");
+
 struct isc_rwlock {
-	alignas(ISC_OS_CACHELINE_SIZE) atomic_uint_fast32_t readers_ingress;
+	atomic_uint_fast32_t readers_ingress;
-	alignas(ISC_OS_CACHELINE_SIZE) atomic_uint_fast32_t readers_egress;
+	uint8_t __padding1[ISC_OS_CACHELINE_SIZE - sizeof(atomic_uint_fast32_t)];
-	alignas(ISC_OS_CACHELINE_SIZE) atomic_int_fast32_t writers_barrier;
+	atomic_uint_fast32_t readers_egress;
-	alignas(ISC_OS_CACHELINE_SIZE) atomic_bool writers_lock;
+	uint8_t __padding2[ISC_OS_CACHELINE_SIZE - sizeof(atomic_uint_fast32_t)];
+	atomic_int_fast32_t writers_barrier;
+	uint8_t __padding3[ISC_OS_CACHELINE_SIZE - sizeof(atomic_int_fast32_t)];
+	atomic_bool writers_lock;
 };
 
 typedef struct isc_rwlock isc_rwlock_t;

lib/isc/job_p.h

@@ -13,7 +13,6 @@
 
 #pragma once
 
-#include <isc/align.h>
 #include <isc/job.h>
 #include <isc/loop.h>
 #include <isc/os.h>
@@ -24,9 +23,15 @@
  * mutex, because we are only using enqueue and splice, and those don't need
  * any synchronization (see urcu/wfcqueue.h for detailed description).
  */
+STATIC_ASSERT(ISC_OS_CACHELINE_SIZE >= sizeof(struct __cds_wfcq_head),
+	      "ISC_OS_CACHELINE_SIZE smaller than "
+	      "sizeof(struct __cds_wfcq_head)");
+
 typedef struct isc_jobqueue {
-	alignas(ISC_OS_CACHELINE_SIZE) struct __cds_wfcq_head head;
+	struct __cds_wfcq_head head;
-	alignas(ISC_OS_CACHELINE_SIZE) struct cds_wfcq_tail tail;
+	uint8_t __padding[ISC_OS_CACHELINE_SIZE -
+			  sizeof(struct __cds_wfcq_head)];
+	struct cds_wfcq_tail tail;
 } isc_jobqueue_t;
 
 typedef ISC_LIST(isc_job_t) isc_joblist_t;

lib/isc/mem.c

@@ -21,7 +21,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include <isc/align.h>
 #include <isc/hash.h>
 #include <isc/magic.h>
 #include <isc/mem.h>