[gdb/symtab] Make per_cu->m_lang atomic

When building gdb with -fsanitize=thread and running test-case
gdb.dwarf2/inlined_subroutine-inheritance.exp, we run into a data race
between:
...
  Read of size 1 at 0x7b2000003010 by thread T4:
    #0 packed<language, 1ul>::operator language() const packed.h:54
    #1 dwarf2_per_cu_data::set_lang(language) read.h:363
...
and:
...
  Previous write of size 1 at 0x7b2000003010 by main thread:
    #0 dwarf2_per_cu_data::set_lang(language) read.h:365
...

Fix this by making per_cu->m_lang atomic.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29286

diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index acbe298b31047fddbb0199680fb72602182ac9f0..c2f86a9d3672732ca2808884d264150f6ad3557a 100644 (file)
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -181,7 +181,7 @@ private:
   std::atomic<packed<dwarf_unit_type, 1>> m_unit_type {(dwarf_unit_type)0};
 
   /* The language of this CU.  */
-  packed<language, LANGUAGE_BYTES> m_lang = language_unknown;
+  std::atomic<packed<language, LANGUAGE_BYTES>> m_lang {language_unknown};
 
 public:
   /* True if this CU has been scanned by the indexer; false if
@@ -351,21 +351,25 @@ public:
 
   enum language lang (bool strict_p = true) const
   {
+    enum language l = m_lang.load ();
     if (strict_p)
-      gdb_assert (m_lang != language_unknown);
-    return m_lang;
+      gdb_assert (l != language_unknown);
+    return l;
   }
 
   void set_lang (enum language lang)
   {
     if (unit_type () == DW_UT_partial)
       return;
-    if (m_lang == language_unknown)
-      /* Set if not set already.  */
-      m_lang = lang;
-    else
-      /* If already set, verify that it's the same value.  */
-      gdb_assert (m_lang == lang);
+    /* Set if not set already.  */
+    packed<language, LANGUAGE_BYTES> nope = language_unknown;
+    if (m_lang.compare_exchange_strong (nope, lang))
+      return;
+    /* If already set, verify that it's the same value.  */
+    nope = lang;
+    if (m_lang.compare_exchange_strong (nope, lang))
+      return;
+    gdb_assert_not_reached ();
   }
 
   /* Free any cached file names.  */