On Wed, Jun 19, 2013 at 01:17:03PM -0700, Ben Pfaff wrote:
> POSIX defines a portable pthread_key_t API for per-thread data. GCC and
> C11 have two different forms of per-thread data that are generally faster
> than the POSIX API, where they are available. This commit adds a
> macro-based wrapper, DEFINE_PER_THREAD_DATA, that takes advantage of the
> GCC extension where it is available and falls back to the POSIX API
> otherwise. (I'm not aware of any compilers that implement the C11 feature,
> so this commit doesn't try to use it.)
Ed Maste pointed out off-list that clang on FreeBSD supports
_Thread_local. Here's a revised version of the patch that supports
both _Thread_local and __thread. I've also updated the "reviews"
branch.
--8<--------------------------cut here-------------------------->8--
From: Ben Pfaff <b...@nicira.com>
Date: Thu, 20 Jun 2013 13:11:26 -0700
Subject: [PATCH] ovs-thread: Add per-thread data support.
POSIX defines a portable pthread_key_t API for per-thread data. GCC and
C11 have two different forms of per-thread data that are generally faster
than the POSIX API, where they are available. This commit adds a
macro-based wrapper, DEFINE_PER_THREAD_DATA, that takes advantage of these
features where they are available and falls back to the POSIX API
otherwise.
The Clang compiler implements the C11 _Thread_local keyword.
This commit also adds a convenience wrapper for the POSIX API, via the
DEFINE_PER_THREAD_MALLOCED_DATA macro.
Signed-off-by: Ben Pfaff <b...@nicira.com>
---
configure.ac | 1 +
lib/ovs-thread.h | 194 +++++++++++++++++++++++++++++++++++++++++++++++++++++
m4/openvswitch.m4 | 37 ++++++++++-
3 files changed, 231 insertions(+), 1 deletions(-)
diff --git a/configure.ac b/configure.ac
index a691963..6c610c1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -80,6 +80,7 @@ OVS_CHECK_XENSERVER_VERSION
OVS_CHECK_GROFF
OVS_CHECK_GNU_MAKE
OVS_CHECK_CACHE_TIME
+OVS_CHECK__THREAD_LOCAL
OVS_ENABLE_OPTION([-Wall])
OVS_ENABLE_OPTION([-Wno-sign-compare])
diff --git a/lib/ovs-thread.h b/lib/ovs-thread.h
index cafeedf..d5bf048 100644
--- a/lib/ovs-thread.h
+++ b/lib/ovs-thread.h
@@ -85,5 +85,199 @@ void xpthread_cond_wait(pthread_cond_t *, pthread_mutex_t
*mutex)
void xpthread_key_create(pthread_key_t *, void (*destructor)(void *));
void xpthread_create(pthread_t *, pthread_attr_t *, void *(*)(void *), void *);
+
+/* Per-thread data.
+ *
+ * Multiple forms of per-thread data exist, each with its own pluses and
+ * minuses:
+ *
+ * - POSIX per-thread data via pthread_key_t is portable to any pthreads
+ * implementation, and allows a destructor function to be defined. It
+ * only (directly) supports per-thread pointers, which are always
+ * initialized to NULL. It requires once-only allocation of a
+ * pthread_key_t value. It is relatively slow.
+ *
+ * - The _Thread_local keyword newly defined in C11 works with any data
+ * type and initializer, and it is fast. _Thread_local does not require
+ * once-only initialization like pthread_key_t. C11 does not define what
+ * happens if one attempts to access a _Thread_local object from a thread
+ * other than the one to which that object belongs. There is no
+ * provision to call a user-specified destructor when a thread ends.
+ *
+ * - The __thread keyword is a GCC extension similar to _Thread_local but
+ * with a longer history. __thread is not portable to every GCC version
+ * or environment. __thread does not restrict the use of a thread-local
+ * object outside its own thread.
+ *
+ * Here's a handy summary:
+ *
+ * pthread_key_t _Thread_local __thread
+ * ------------- ------------- -------------
+ * portability high low medium
+ * speed low high high
+ * supports destructors? yes no no
+ * needs key allocation? yes no no
+ * arbitrary initializer? no yes yes
+ * cross-thread access? yes no yes
+ */
+
+/* DEFINE_PER_THREAD_DATA(TYPE, NAME, INITIALIZER).
+ *
+ * One should prefer to use POSIX per-thread data, via pthread_key_t, when its
+ * performance is acceptable, because of its portability (see the table above).
+ * This macro is an alternatives that takes advantage of _Thread_local (and
+ * __thread, via a config.h macro substitution), for its performance, when it
+ * is available, and falls back to POSIX per-thread data otherwise.
+ *
+ * Defines per-thread variable NAME with the given TYPE, initialized to
+ * INITIALIZER (which must be valid as an initializer for a variable with
+ * static lifetime).
+ *
+ * The public interface to the variable is:
+ *
+ * TYPE *NAME_get(void)
+ * TYPE *NAME_get__(void)
+ *
+ * Returns the address of this thread's instance of NAME.
+ *
+ * Use NAME_get() in a context where this might be the first use of the
+ * per-thread variable in the program. Use NAME_get__(), which avoids a
+ * conditional test and is thus slightly faster, in a context where one
+ * knows that NAME_get() has already been called previously.
+ *
+ * There are no "NAME_set()" or "NAME_set__()" functions. To set the value of
+ * the per-thread variable, dereference the pointer returned by TYPE_get() or
+ * TYPE_get__(), e.g. *TYPE_get() = 0.
+ */
+#if HAVE__THREAD_LOCAL
+#define DEFINE_PER_THREAD_DATA(TYPE, NAME, ...) \
+ typedef TYPE NAME##_type; \
+ static _Thread_local NAME##_type NAME##_var = __VA_ARGS__; \
+ \
+ static NAME##_type * \
+ NAME##_get__(void) \
+ { \
+ return &NAME##_var; \
+ } \
+ \
+ static NAME##_type * \
+ NAME##_get(void) \
+ { \
+ return NAME##_get__(); \
+ }
+#else
+#define DEFINE_PER_THREAD_DATA(TYPE, NAME, ...) \
+ typedef TYPE NAME##_type; \
+ static pthread_key_t NAME##_key; \
+ \
+ static NAME##_type * \
+ NAME##_get__(void) \
+ { \
+ return pthread_getspecific(NAME##_key); \
+ } \
+ \
+ static void \
+ NAME##_once_init(void) \
+ { \
+ if (pthread_key_create(&NAME##_key, free)) { \
+ abort(); \
+ } \
+ } \
+ \
+ static NAME##_type * \
+ NAME##_get(void) \
+ { \
+ static pthread_once_t once = PTHREAD_ONCE_INIT; \
+ NAME##_type *value; \
+ \
+ pthread_once(&once, NAME##_once_init); \
+ value = NAME##_get__(); \
+ if (!value) { \
+ static const NAME##_type initial_value = __VA_ARGS__; \
+ \
+ value = xmalloc(sizeof *value); \
+ *value = initial_value; \
+ pthread_setspecific(NAME##_key, value); \
+ } \
+ return value; \
+ }
+#endif
+
+/* DEFINE_PER_THREAD_MALLOCED_DATA(TYPE, NAME).
+ *
+ * This is a simple wrapper around POSIX per-thread data primitives. It
+ * defines per-thread variable NAME with the given TYPE, which must be a
+ * pointer type. In each thread, the per-thread variable is initialized to
+ * NULL. When a thread terminates, the variable is freed with free().
+ *
+ * The public interface to the variable is:
+ *
+ * TYPE NAME_get(void)
+ * TYPE NAME_get__(void)
+ *
+ * Returns the value of per-thread variable NAME in this thread.
+ *
+ * Use NAME_get() in a context where this might be the first use of the
+ * per-thread variable in the program. Use NAME_get__(), which avoids a
+ * conditional test and is thus slightly faster, in a context where one
+ * knows that NAME_get() has already been called previously.
+ *
+ * TYPE NAME_set(TYPE new_value)
+ * TYPE NAME_set__(TYPE new_value)
+ *
+ * Sets the value of per-thread variable NAME to 'new_value' in this
+ * thread, and returns its previous value.
+ *
+ * Use NAME_set() in a context where this might be the first use of the
+ * per-thread variable in the program. Use NAME_set__(), which avoids a
+ * conditional test and is thus slightly faster, in a context where one
+ * knows that NAME_set() has already been called previously.
+ */
+#define DEFINE_PER_THREAD_MALLOCED_DATA(TYPE, NAME) \
+ static pthread_key_t NAME##_key; \
+ \
+ static void \
+ NAME##_once_init(void) \
+ { \
+ if (pthread_key_create(&NAME##_key, free)) { \
+ abort(); \
+ } \
+ } \
+ \
+ static void \
+ NAME##_init(void) \
+ { \
+ static pthread_once_t once = PTHREAD_ONCE_INIT; \
+ pthread_once(&once, NAME##_once_init); \
+ } \
+ \
+ static TYPE \
+ NAME##_get__(void) \
+ { \
+ return pthread_getspecific(NAME##_key); \
+ } \
+ \
+ static OVS_UNUSED TYPE \
+ NAME##_get(void) \
+ { \
+ NAME##_init(); \
+ return NAME##_get__(); \
+ } \
+ \
+ static TYPE \
+ NAME##_set__(TYPE value) \
+ { \
+ TYPE old_value = NAME##_get__(); \
+ pthread_setspecific(NAME##_key, value); \
+ return old_value; \
+ } \
+ \
+ static OVS_UNUSED TYPE \
+ NAME##_set(TYPE value) \
+ { \
+ NAME##_init(); \
+ return NAME##_set__(value); \
+ }
+
#endif /* ovs-thread.h */
diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
index 12c02c0..3895346 100644
--- a/m4/openvswitch.m4
+++ b/m4/openvswitch.m4
@@ -1,6 +1,6 @@
# -*- autoconf -*-
-# Copyright (c) 2008, 2009, 2010, 2011, 2012 Nicira, Inc.
+# Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013 Nicira, Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
@@ -390,3 +390,38 @@ AC_DEFUN([OVS_CHECK_GROFF],
ovs_cv_groff=no
fi])
AM_CONDITIONAL([HAVE_GROFF], [test "$ovs_cv_groff" = yes])])
+
+dnl OVS_CHECK__THREAD_LOCAL
+dnl
+dnl Checks whether the compiler and linker support the C11
+dnl _Thread_local storage class or the GCC __thread extension, and if
+dnl so define HAVE__THREAD_LOCAL. If __thread is supported, also #defines
+dnl _Thread_local to __thread to hide the difference.
+AC_DEFUN([OVS_CHECK__THREAD_LOCAL],
+ [AC_CACHE_CHECK(
+ [whether $CC supports _Thread_local],
+ [ovs_cv__Thread_local],
+ [AC_LINK_IFELSE(
+ [AC_LANG_PROGRAM([static _Thread_local var;], [return var;])],
+ [ovs_cv__Thread_local=yes],
+ [ovs_cv__Thread_local=no])])
+ if test $ovs_cv__Thread_local = no; then
+ AC_CACHE_CHECK(
+ [whether $CC supports __thread],
+ [ovs_cv___thread],
+ [AC_LINK_IFELSE(
+ [AC_LANG_PROGRAM([static __thread var;], [return var;])],
+ [ovs_cv___thread=yes],
+ [ovs_cv___thread=no])])
+ if test $ovs_cv___thread = yes; then
+ AC_DEFINE([_Thread_local], [__thread],
+ [Define _Thread_local to use GCC __thread extension.])
+ fi
+ fi
+ if test $ovs_cv__Thread_local = yes || test $ovs_cv___thread = yes; then
+ AC_DEFINE(
+ [HAVE__THREAD_LOCAL],
+ [1],
+ [Define to 1 if the C compiler and linker support the C11 _Thread_local
+ storage class or the compatible GCC __thread extension.])
+ fi])
--
1.7.2.5
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