[OpenMP] Insert missing variable update inside loop

[jlpeyton]

Summary

A loop within task priority code did not have necessary update of ntasks variable which leads to hangs if two threads collide when both attempt to execute the __kmp_atomic_compare_store()

Fixes: https://github.com/llvm/llvm-project/issues/62867

openmp/runtime/src/kmp_tasking.cpp

KA_TRACE(
         20, ("__kmp_get_priority_task(exit #1): T#%d No tasks to get\n", gtid));
     return NULL;
   }
   do {
     // decrement num_tasks to "reserve" one task to get for execution
     if (__kmp_atomic_compare_store(&task_team->tt.tt_num_task_pri, ntasks,
                                    ntasks - 1))
       break;
+    ntasks = task_team->tt.tt_num_task_pri;
   } while (ntasks > 0);
   if (ntasks == 0) {
     KA_TRACE(20, ("__kmp_get_priority_task(exit #2): T#%d No tasks to get\n",
                   __kmp_get_gtid()));
     return NULL;
   }
   // We got a "ticket" to get a "reserved" priority task
   int deque_ntasks;

openmp/runtime/test/tasking/omp_task_priority3.c

+// RUN: %libomp-compile && env OMP_MAX_TASK_PRIORITY=42 %libomp-run
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <omp.h>
+
+int a = 0;
+
+int main(void) {
+  int i;
+  int max_task_priority = omp_get_max_task_priority();
+  if (max_task_priority != 42) {
+    fprintf(stderr,
+            "error: omp_get_max_task_priority() returned %d instead of 42\n",
+            max_task_priority);
+    exit(EXIT_FAILURE);
+  }
+
+  for (i = 0; i < 250; ++i) {
+    #pragma omp parallel
+    {
+      #pragma omp task priority(42)
+      {
+        #pragma omp atomic
+        a++;
+      }
+    }
+  }
+
+  printf("a = %d\n", a);
+
+  return EXIT_SUCCESS;
+}