[Mesa-dev] [PATCH] c11/threads: create mutexattrs only when needed

[Emil Velikov]

From: Emil Velikov <emil.velikov at collabora.com>

If the mutexattrs are the default one can just pass NULL to
pthread_mutex_init. As the compiler does not know this detail it
unnecessarily creates/destroys the attrs.

Signed-off-by: Emil Velikov <emil.velikov at collabora.com>
---
While going through GLVND, I've noticed that it (sort of) breaks its
assumptions/goals - 'we don't want the heavy locking/etc. brought by
pthreads' [for single threaded uses]

Thus I gave mesa a quick look and the following popped up:

- pthread_once - libglapi, classic + dri
Replace with an atomic test & set combo ?

- pthread_mutexattr_* - all dri modules, libGL-apple
Using a recursive lock in src/mesa/main/shared.c and
src/glx/apple/apple_glx_drawable.c

- pthread_key_* - EGL
- pthread_.etspecific - EGL
Extend pthread-stubs explicitly required it by mesa ?
Note: the original code that pthread-stubs is based on (libX11) does
have these ;-)

- pthread_barrier_* - llvmpipe
Fall-back to the mutex + cond implementation ?

- pthread_setname_np - llvmpipe
Do we need this ? Afaict the Windows build does not have an equivalent.

- pthread_join - nine, llvmpipe, radeon(s), rbug, omx (thanks bellagio)
- pthread_create - nine, llvmpipe, radeon(s), rbug
- pthread_sigmask - nine, llvmpipe, radeon(s), rbug

These four (five inc bellagio/omx) want more than one thread. How do we
get others pthread free, while keeping these happy ?

Please let me know how you feel on the topic. Do you see this as worthy
goal ? Does the proposed solutions sound OK ? Can you think of any
alternatives?

-Emil

P.S. For anyone who wonders, libc (GNU one only iirc) provides
lightweight stubs, thus single-threaded apps work without the overhead.
---
 include/c11/threads_posix.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/c11/threads_posix.h b/include/c11/threads_posix.h
index ce9853b..11d36e4 100644
--- a/include/c11/threads_posix.h
+++ b/include/c11/threads_posix.h
@@ -180,9 +180,14 @@ mtx_init(mtx_t *mtx, int type)
       && type != (mtx_timed|mtx_recursive)
       && type != (mtx_try|mtx_recursive))
         return thrd_error;
+
+    if ((type & mtx_recursive) == 0) {
+        pthread_mutex_init(mtx, NULL);
+        return thrd_success;
+    }
+
     pthread_mutexattr_init(&attr);
-    if ((type & mtx_recursive) != 0)
-        pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
     pthread_mutex_init(mtx, &attr);
     pthread_mutexattr_destroy(&attr);
     return thrd_success;
-- 
2.8.0