Re: [PATCH 27/36] attr: convert to new threadsafe API

On Wed, Oct 26, 2016 at 5:15 AM, Jeff King [off-list ref] wrote:

On Wed, Oct 26, 2016 at 11:35:58AM +0200, Simon Ruderich wrote:

quoted

quoted

 static pthread_mutex_t attr_mutex;
-#define attr_lock()pthread_mutex_lock(&attr_mutex)
+static inline void attr_lock(void)
+{
+   static int initialized;
+
+   if (!initialized) {
+           pthread_mutex_init(&attr_mutex, NULL);
+           initialized = 1;
+   }
+   pthread_mutex_lock(&attr_mutex);
+}

This may initialize the mutex multiple times during the first
lock (which may happen in parallel).

pthread provides static initializers. To quote the man page:

    Variables of type pthread_mutex_t can also be initialized
    statically, using the constants PTHREAD_MUTEX_INITIALIZER
    (for fast mutexes), PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP
    (for recursive mutexes), and
    PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP (for error checking
    mutexes).

I seem to recall this does not work on Windows, where the pthread
functions are thin wrappers over CRITICAL_SECTION. Other threaded code
in git does an explicit setup step before entering threaded sections.
E.g., see start_threads() in builtin/grep.c.

I wonder if we can have a similar thing as
http://stackoverflow.com/a/9490113 in compat/win32/pthread.{h.c} as it is
very convenient to not have to explicitly initialize mutexes?