Fix -fsanitize=thread for per_cu fields

When building gdb with -fsanitize=thread and gcc 12, and running test-case
gdb.dwarf2/dwz.exp, we run into a data race between:
...
  Read of size 1 at 0x7b200000300d by thread T2:^M
    #0 cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, \
    abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) gdb/dwarf2/read.c:6164 \
    (gdb+0x82ec95)^M
...
and:
...
  Previous write of size 1 at 0x7b200000300d by main thread:^M
    #0 prepare_one_comp_unit gdb/dwarf2/read.c:23588 (gdb+0x86f973)^M
...

In other words, between:
...
  if (this_cu->reading_dwo_directly)
...
and:
...
    cu->per_cu->lang = pretend_language;
...

Likewise, we run into a data race between:
...
  Write of size 1 at 0x7b200000300e by thread T4:
    #0 process_psymtab_comp_unit gdb/dwarf2/read.c:6789 (gdb+0x830720)
...
and:
...
  Previous read of size 1 at 0x7b200000300e by main thread:
    #0 cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, \
    abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) gdb/dwarf2/read.c:6164 \
    (gdb+0x82edab)
...

In other words, between:
...
      this_cu->unit_type = DW_UT_partial;
...
and:
...
  if (this_cu->reading_dwo_directly)
...

Likewise for the write to addresses_seen in cooked_indexer::check_bounds and a
read from is_dwz in dwarf2_find_containing_comp_unit for test-case
gdb.dwarf2/dw2-dir-file-name.exp and target board cc-with-dwz-m.

The problem is that the written fields are part of the same memory location as
the read fields, so executing a read and write in different threads is
undefined behavour.

Making the written fields separate memory locations, using the new
struct packed template fixes this.

The set of fields has been established experimentally to be the
minimal set to get rid of this type of -fsanitize=thread errors, but
more fields might require the same treatment.

Looking at the properties of the lang field, unlike dwarf_version it's
not available in the unit header, so it will be set the first time
during the parallel cooked index reading.  The same holds for
unit_type, and likewise for addresses_seen.

dwarf2_per_cu_data::addresses_seen is moved so that the bitfields that
currently follow it can be merged in the same memory location as the
bitfields that currently precede it, for better packing.

Tested on x86_64-linux.

Co-Authored-By: Pedro Alves <pedro@palves.net>

Change-Id: Ifa94f0a2cebfae5e8f6ddc73265f05e7fd9e1532

1 files changed, 11 insertions, 9 deletions

diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 5e0ce5e..707dca9 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -33,6 +33,7 @@
 #include "gdbsupport/gdb_obstack.h"
 #include "gdbsupport/hash_enum.h"
 #include "gdbsupport/function-view.h"
+#include "gdbsupport/packed.h"
 
 /* Hold 'maintenance (set|show) dwarf' commands.  */
 extern struct cmd_list_element *set_dwarf_cmdlist;
@@ -105,11 +106,8 @@ struct dwarf2_per_cu_data
       reading_dwo_directly (false),
       tu_read (false),
       m_header_read_in (false),
-      addresses_seen (false),
       mark (false),
       files_read (false),
-      m_unit_type {},
-      m_lang (language_unknown),
       scanned (false)
   {
   }
@@ -162,10 +160,6 @@ public:
      it private at the moment.  */
   mutable bool m_header_read_in : 1;
 
-  /* If addresses have been read for this CU (usually from
-     .debug_aranges), then this flag is set.  */
-  bool addresses_seen : 1;
-
   /* A temporary mark bit used when iterating over all CUs in
      expand_symtabs_matching.  */
   unsigned int mark : 1;
@@ -174,12 +168,20 @@ public:
      point in trying to read it again next time.  */
   bool files_read : 1;
 
+  /* Wrap the following in struct packed instead of bitfields to avoid
+     data races when the bitfields end up on the same memory location
+     (per C++ memory model).  */
+
+  /* If addresses have been read for this CU (usually from
+     .debug_aranges), then this flag is set.  */
+  packed<bool, 1> addresses_seen = false;
+
 private:
   /* The unit type of this CU.  */
-  ENUM_BITFIELD (dwarf_unit_type) m_unit_type : 8;
+  packed<dwarf_unit_type, 1> m_unit_type = (dwarf_unit_type) 0;
 
   /* The language of this CU.  */
-  ENUM_BITFIELD (language) m_lang : LANGUAGE_BITS;
+  packed<language, LANGUAGE_BYTES> m_lang = language_unknown;
 
 public:
   /* True if this CU has been scanned by the indexer; false if