[PATCH, libgomp] Fix deadlock in acc_set_device_type

[Chung-Lin Tang]

Hi Jakub, there's a path for deadlock on acc_device_lock when going
through the acc_set_device_type() OpenACC library function.
Basically, the gomp_init_targets_once() function should not be
called with that held. The attached patch moves it appropriately.

Also in this patch, there are several cases in acc_* functions
where gomp_init_targets_once() is guarded by a test of
!cached_base_dev. Since that function already uses pthread_once() to
call gomp_target_init(), and technically cached_base_dev
is protected by acc_device_lock, the cleanest way should be to
simply drop those "if(!cached_base_dev)" tests.

Tested libgomp without regressions on an nvptx offloaded system,
is this okay for trunk?

Thanks,
Chung-Lin

2016-03-28  Chung-Lin Tang  <clt...@codesourcery.com>

        * oacc-init.c (acc_init): Remove !cached_base_dev condition on call to
        gomp_init_targets_once().
        (acc_set_device_type): Remove !cached_base_dev condition on call to
        gomp_init_targets_once(), move call to before acc_device_lock acquire,
        to avoid deadlock.
        (acc_get_device_num): Remove !cached_base_dev condition on call to
        gomp_init_targets_once().
        (acc_set_device_num): Likewise.

Index: oacc-init.c
===================================================================
--- oacc-init.c (revision 234502)
+++ oacc-init.c (working copy)
@@ -433,8 +433,7 @@ goacc_attach_host_thread_to_device (int ord)
 void
 acc_init (acc_device_t d)
 {
-  if (!cached_base_dev)
-    gomp_init_targets_once ();
+  gomp_init_targets_once ();
 
   gomp_mutex_lock (&acc_device_lock);
 
@@ -498,11 +497,10 @@ acc_set_device_type (acc_device_t d)
   struct gomp_device_descr *base_dev, *acc_dev;
   struct goacc_thread *thr = goacc_thread ();
 
+  gomp_init_targets_once ();
+
   gomp_mutex_lock (&acc_device_lock);
 
-  if (!cached_base_dev)
-    gomp_init_targets_once ();
-
   cached_base_dev = base_dev = resolve_device (d, true);
   acc_dev = &base_dev[goacc_device_num];
 
@@ -563,8 +561,7 @@ acc_get_device_num (acc_device_t d)
   if (d >= _ACC_device_hwm)
     gomp_fatal ("unknown device type %u", (unsigned) d);
 
-  if (!cached_base_dev)
-    gomp_init_targets_once ();
+  gomp_init_targets_once ();
 
   gomp_mutex_lock (&acc_device_lock);
   dev = resolve_device (d, true);
@@ -584,8 +581,7 @@ acc_set_device_num (int ord, acc_device_t d)
   struct gomp_device_descr *base_dev, *acc_dev;
   int num_devices;
 
-  if (!cached_base_dev)
-    gomp_init_targets_once ();
+  gomp_init_targets_once ();
 
   if (ord < 0)
     ord = goacc_device_num;