libgomp: Fix hang when profiling OpenACC programs with CUDA 9.0 nvprof

The version of nvprof in CUDA 9.0 causes a hang when used to profile an
OpenACC program.  This is because it calls acc_get_device_type from
a callback called during device initialization, which then attempts
to acquire acc_device_lock while it is already taken, resulting in
deadlock.  This works around the issue by returning acc_device_none
from acc_get_device_type without attempting to acquire the lock when
initialization has not completed yet.

2020-07-14  Tom de Vries  <tom@codesourcery.com>
	    Cesar Philippidis  <cesar@codesourcery.com>
	    Thomas Schwinge  <thomas@codesourcery.com>
	    Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgomp/
	* oacc-init.c (acc_init_state_lock, acc_init_state, acc_init_thread):
	New variable.
	(acc_init_1): Set acc_init_thread to pthread_self ().  Set
	acc_init_state to initializing at the start, and to initialized at the
	end.
	(self_initializing_p): New function.
	(acc_get_device_type): Return acc_device_none if called by thread that
	is currently executing acc_init_1.
	* libgomp.texi (acc_get_device_type): Update documentation.
	(Implementation Status and Implementation-Defined Behavior): Likewise.
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-init-2.c: New.

libgomp/libgomp.texi

@@ -1967,6 +1967,12 @@ in @var{devicetype}, to use when executing a parallel or kernels region.
 This function returns what device type will be used when executing a
 parallel or kernels region.
 
+This function returns @code{acc_device_none} if
+@code{acc_get_device_type} is called from
+@code{acc_ev_device_init_start}, @code{acc_ev_device_init_end}
+callbacks of the OpenACC Profiling Interface (@ref{OpenACC Profiling
+Interface}), that is, if the device is currently being initialized.
+
 @item @emph{C/C++}:
 @multitable @columnfractions .20 .80
 @item @emph{Prototype}: @tab @code{acc_device_t acc_get_device_type(void);}
@@ -3382,6 +3388,11 @@ every event that has been registered.
 
 We're not yet accounting for the fact that @cite{OpenACC events may
 occur during event processing}.
+We just handle one case specially, as required by CUDA 9.0
+@command{nvprof}, that @code{acc_get_device_type}
+(@ref{acc_get_device_type})) may be called from
+@code{acc_ev_device_init_start}, @code{acc_ev_device_init_end}
+callbacks.
 
 We're not yet implementing initialization via a
 @code{acc_register_library} function that is either statically linked

libgomp/oacc-init.c

@@ -40,6 +40,11 @@
 
 static gomp_mutex_t acc_device_lock;
 
+static gomp_mutex_t acc_init_state_lock;
+static enum { uninitialized, initializing, initialized } acc_init_state
+  = uninitialized;
+static pthread_t acc_init_thread;
+
 /* A cached version of the dispatcher for the global "current" accelerator type,
    e.g. used as the default when creating new host threads.  This is the
    device-type equivalent of goacc_device_num (which specifies which device to
@@ -228,6 +233,11 @@ acc_dev_num_out_of_range (acc_device_t d, int ord, int ndevs)
 static struct gomp_device_descr *
 acc_init_1 (acc_device_t d, acc_construct_t parent_construct, int implicit)
 {
+  gomp_mutex_lock (&acc_init_state_lock);
+  acc_init_state = initializing;
+  acc_init_thread = pthread_self ();
+  gomp_mutex_unlock (&acc_init_state_lock);
+
   bool check_not_nested_p;
   if (implicit)
     {
@@ -317,6 +327,14 @@ acc_init_1 (acc_device_t d, acc_construct_t parent_construct, int implicit)
 				&api_info);
     }
 
+  /* We're setting 'initialized' *after* 'goacc_profiling_dispatch', so that a
+     nested 'acc_get_device_type' called from a profiling callback still sees
+     'initializing', so that we don't deadlock when it then again tries to lock
+     'goacc_prof_lock'.  See also the discussion in 'acc_get_device_type'.  */
+  gomp_mutex_lock (&acc_init_state_lock);
+  acc_init_state = initialized;
+  gomp_mutex_unlock (&acc_init_state_lock);
+
   return base_dev;
 }
 
@@ -643,6 +661,17 @@ acc_set_device_type (acc_device_t d)
 
 ialias (acc_set_device_type)
 
+static bool
+self_initializing_p (void)
+{
+  bool res;
+  gomp_mutex_lock (&acc_init_state_lock);
+  res = (acc_init_state == initializing
+	 && pthread_equal (acc_init_thread, pthread_self ()));
+  gomp_mutex_unlock (&acc_init_state_lock);
+  return res;
+}
+
 acc_device_t
 acc_get_device_type (void)
 {
@@ -652,6 +681,15 @@ acc_get_device_type (void)
 
   if (thr && thr->base_dev)
     res = acc_device_type (thr->base_dev->type);
+  else if (self_initializing_p ())
+    /* The Cuda libaccinj64.so version 9.0+ calls acc_get_device_type during the
+       acc_ev_device_init_start event callback, which is dispatched during
+       acc_init_1.  Trying to lock acc_device_lock during such a call (as we do
+       in the else clause below), will result in deadlock, since the lock has
+       already been taken by the acc_init_1 caller.  We work around this problem
+       by using the acc_get_device_type property "If the device type has not yet
+       been selected, the value acc_device_none may be returned".  */
+    ;
   else
     {
       acc_prof_info prof_info;

libgomp/testsuite/libgomp.oacc-c-c++-common/acc_prof-init-2.c

@@ -0,0 +1,80 @@
+/* { dg-do run } */
+/* { dg-timeout 10 } */
+
+/* Test the calling of 'acc_get_device_type' from within
+   'cb_device_init_start' and 'cb_device_init_end' callbacks.  This occurs
+   when the CUDA 9.0 'nvprof' tool is used, and previously deadlocked.  */
+
+#include <assert.h>
+#include <stdbool.h>
+#include <acc_prof.h>
+
+static acc_prof_reg reg;
+static acc_prof_reg unreg;
+static acc_prof_lookup_func lookup;
+
+void acc_register_library (acc_prof_reg reg_, acc_prof_reg unreg_, acc_prof_lookup_func lookup_)
+{
+  reg = reg_;
+  unreg = unreg_;
+  lookup = lookup_;
+}
+
+static bool expect_cb_device_init_start;
+static bool expect_cb_device_init_end;
+
+static void cb_device_init_start (acc_prof_info *prof_info, acc_event_info *event_info, acc_api_info *api_info)
+{
+  assert (expect_cb_device_init_start);
+  expect_cb_device_init_start = false;
+
+  acc_device_t acc_device_type;
+  acc_device_type = acc_get_device_type ();
+  assert (acc_device_type == acc_device_none);
+
+  expect_cb_device_init_end = true;
+}
+
+static void cb_device_init_end (acc_prof_info *prof_info, acc_event_info *event_info, acc_api_info *api_info)
+{
+  assert (expect_cb_device_init_end);
+  expect_cb_device_init_end = false;
+
+  acc_device_t acc_device_type;
+  acc_device_type = acc_get_device_type ();
+  assert (acc_device_type == acc_device_none);
+}
+
+int main(void)
+{
+  acc_register_library (acc_prof_register, acc_prof_unregister, acc_prof_lookup);
+
+  reg (acc_ev_device_init_start, cb_device_init_start, acc_reg);
+  reg (acc_ev_device_init_end, cb_device_init_end, acc_reg);
+
+  expect_cb_device_init_start = true;
+  expect_cb_device_init_end = false;
+  acc_init (acc_device_host);
+  assert (!expect_cb_device_init_start);
+  assert (!expect_cb_device_init_end);
+  {
+    acc_device_t acc_device_type;
+    acc_device_type = acc_get_device_type ();
+    assert (acc_device_type == acc_device_host);
+  }
+  acc_shutdown (acc_device_host);
+
+  expect_cb_device_init_start = true;
+  expect_cb_device_init_end = false;
+  acc_init (acc_device_default);
+  assert (!expect_cb_device_init_start);
+  assert (!expect_cb_device_init_end);
+  {
+    acc_device_t acc_device_type;
+    acc_device_type = acc_get_device_type ();
+    assert (acc_device_type != acc_device_none);
+  }
+  acc_shutdown (acc_device_default);
+
+  return 0;
+}