[PATCH][gdbsupport] Avoid main thread in parallel for with thread sanitizer

[Tom de Vries]

Hi,

When running tasks in a parallel for, some elements may be handled by a worker
thread and others by the main thread.

When running with -fsanitize=thread, avoid the main thread in the parallel
for, to prevent multi-threading problems being hidden.

Tested on x86_64-linux, with -fsanitize=thread.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29359

Any comments?

Thanks,
- Tom

[gdbsupport] Avoid main thread in parallel for with thread sanitizer

---
 gdbsupport/parallel-for.h | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/gdbsupport/parallel-for.h b/gdbsupport/parallel-for.h
index bf40f125f0f..81a2cfa215b 100644
--- a/gdbsupport/parallel-for.h
+++ b/gdbsupport/parallel-for.h
@@ -24,6 +24,15 @@
 #include <type_traits>
 #include "gdbsupport/thread-pool.h"
 
+/* GCC does not understand __has_feature.  */
+#if !defined(__has_feature)
+# define __has_feature(x) 0
+#endif
+
+#if __has_feature(thread_sanitizer)
+#define __SANITIZE_ADDRESS__ 1
+#endif
+
 namespace gdb
 {
 
@@ -161,7 +170,22 @@ parallel_for_each (unsigned n, RandomIt first, RandomIt last,
       /* n_elements == n_threads * elts_per_thread + elts_left_over. */
     }
 
-  size_t count = n_threads == 0 ? 0 : n_threads - 1;
+  /* Number of worker threads we're going to use.  */
+  size_t count;
+#ifdef __SANITIZE_THREAD__
+  /* Avoid the main thread.  */
+  count = (n_threads == 0
+	   ? (/* Only use the first worker thread.  */
+	      1)
+	   : (/* Only use worker threads.  */
+	      n_threads));
+#else
+  count = (n_threads == 0
+	   ? (/* Only use the main thread.  */
+	      0)
+	   : (/* Leave at least one worker thread unused.  */
+	      n_threads - 1));
+#endif
   gdb::detail::par_for_accumulator<result_type> results (count);
 
   if (parallel_for_each_debug)