check pthread_atfork and thread-local storage

Add pthread_atfork check to configure.ac and uses it when necessary,
Introduces tls.m4 macro for checking thread-local storage support, Puts
values array into thread-local storage
(lxc_global_config_value@src/lxc/utils.c), Removes
static_lock/static_unlock from LXC code.

Lastly, it introduces a warning for bionic users about multithreaded
usage of LXC.

(requires 64b1be2903 to be reverted first)

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>

config/tls.m4

@@ -0,0 +1,14 @@
+# See if we have working TLS.  We only check to see if it compiles, and that
+# the resulting program actually runs, not whether the resulting TLS variables
+# work properly; that check is done at runtime, since we can run binaries
+# compiled with __thread on systems without TLS.
+AC_DEFUN([LXC_CHECK_TLS],
+[
+    AC_MSG_CHECKING(for TLS)
+    AC_RUN_IFELSE([AC_LANG_SOURCE([[ static __thread int val; int main() { return 0; } ]])],[have_tls=yes],[have_tls=no],[have_tls=no ])
+    AC_MSG_RESULT($have_tls)
+    if test "$have_tls" = "yes"; then
+        AC_DEFINE([HAVE_TLS],[1],[Define if the compiler supports __thread])
+        AC_DEFINE([thread_local],[__thread],[Define to the compiler TLS keyword])
+    fi
+])

configure.ac

@@ -483,6 +483,8 @@ AC_CHECK_HEADERS([sys/signalfd.h pty.h ifaddrs.h sys/capability.h sys/personalit
 AC_CHECK_FUNCS([setns pivot_root sethostname unshare rand_r confstr])
 
 # Check for some functions
+AC_CHECK_LIB(pthread, main)
+AC_CHECK_FUNCS(pthread_atfork)
 AC_CHECK_LIB(util, openpty)
 AC_CHECK_FUNCS([openpty hasmntopt setmntent endmntent])
 AC_CHECK_FUNCS([getline],
@@ -502,6 +504,9 @@ AC_SEARCH_LIBS(clock_gettime, [rt])
 AC_PROG_GCC_TRADITIONAL
 AC_PROG_SED
 
+# See if we support thread-local storage.
+LXC_CHECK_TLS
+
 if test "x$GCC" = "xyes"; then
 	CFLAGS="$CFLAGS -Wall -Werror"
 fi
@@ -680,3 +685,13 @@ Debugging:
 Paths:
  - Logs in configpath: $enable_configpath_log
 EOF
+
+if test "x$ac_cv_func_pthread_atfork" = "xno" ; then
+cat << EOF
+
+WARNING: Threading not supported on your platform
+
+	You are compiling LXC for bionic target which lacks certain threading related functionality used by LXC API (like pthread_atfork).
+	Please note that, because of the missing functionality, multithreaded usage of LXC API cause some problems.
+EOF
+fi

src/lxc/lxclock.c

@@ -46,7 +46,6 @@ lxc_log_define(lxc_lock, lxc);
 
 #ifdef MUTEX_DEBUGGING
 static pthread_mutex_t thread_mutex = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;
-static pthread_mutex_t static_mutex = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;
 
 static inline void dump_stacktrace(void)
 {
@@ -68,7 +67,6 @@ static inline void dump_stacktrace(void)
 }
 #else
 static pthread_mutex_t thread_mutex = PTHREAD_MUTEX_INITIALIZER;
-static pthread_mutex_t static_mutex = PTHREAD_MUTEX_INITIALIZER;
 
 static inline void dump_stacktrace(void) {;}
 #endif
@@ -326,28 +324,13 @@ void process_unlock(void)
  * to unlock the mutex.
  * This forbids doing fork() while explicitly holding the lock.
  */
+#ifdef HAVE_PTHREAD_ATFORK
 __attribute__((constructor))
 static void process_lock_setup_atfork(void)
 {
 	pthread_atfork(process_lock, process_unlock, process_unlock);
 }
-
-/* Protects static const values inside the lxc_global_config_value funtion */
-void static_lock(void)
-{
-	lock_mutex(&static_mutex);
-}
-
-void static_unlock(void)
-{
-	unlock_mutex(&static_mutex);
-}
-
-__attribute__((constructor))
-static void static_lock_setup_atfork(void)
-{
-	pthread_atfork(static_lock, static_unlock, static_unlock);
-}
+#endif
 
 int container_mem_lock(struct lxc_container *c)
 {

src/lxc/lxclock.h

@@ -123,16 +123,6 @@ extern void process_lock(void);
  */
 extern void process_unlock(void);
 
-/*!
- * \brief Lock global data.
- */
-extern void static_lock(void);
-
-/*!
- * \brief Unlock global data.
- */
-extern void static_unlock(void);
-
 struct lxc_container;
 
 /*!

src/lxc/utils.c

@@ -250,9 +250,12 @@ static const char *lxc_global_config_value(const char *option_name)
 		{ NULL, NULL },
 	};
 
-	/* Protected by a mutex to eliminate conflicting load and store operations */
+	/* placed in the thread local storage pool for non-bionic targets */	
+#ifdef HAVE_TLS
+	static __thread const char *values[sizeof(options) / sizeof(options[0])] = { 0 };
+#else
 	static const char *values[sizeof(options) / sizeof(options[0])] = { 0 };
-
+#endif
 	char *user_config_path = NULL;
 	char *user_lxc_path = NULL;
 
@@ -273,7 +276,6 @@ static const char *lxc_global_config_value(const char *option_name)
 	}
 
 	const char * const (*ptr)[2];
-	const char *value;
 	size_t i;
 	char buf[1024], *p, *p2;
 	FILE *fin = NULL;
@@ -289,15 +291,11 @@ static const char *lxc_global_config_value(const char *option_name)
 		return NULL;
 	}
 
-	static_lock();
 	if (values[i]) {
 		free(user_config_path);
 		free(user_lxc_path);
-		value = values[i];
-		static_unlock();
-		return value;
+		return values[i];
 	}
-	static_unlock();
 
 	fin = fopen_cloexec(user_config_path, "r");
 	free(user_config_path);
@@ -333,15 +331,12 @@ static const char *lxc_global_config_value(const char *option_name)
 			while (*p && (*p == ' ' || *p == '\t')) p++;
 			if (!*p)
 				continue;
-			static_lock();
 			values[i] = copy_global_config_value(p);
-			static_unlock();
 			free(user_lxc_path);
 			goto out;
 		}
 	}
 	/* could not find value, use default */
-	static_lock();
 	if (strcmp(option_name, "lxcpath") == 0)
 		values[i] = user_lxc_path;
 	else {
@@ -353,16 +348,12 @@ static const char *lxc_global_config_value(const char *option_name)
 	 * as an error... */
 	if (!values[i])
 		errno = 0;
-	static_unlock();
 
 out:
 	if (fin)
 		fclose(fin);
 
-	static_lock();
-	value = values[i];
-	static_unlock();
-	return value;
+	return values[i];
 }
 
 const char *default_lvm_vg(void)