[PATCH 2/5] [gdb/symtab] Fix data race on per_cu->dwarf_version

[Tom de Vries <tdevries@suse.de>]

When building gdb with -fsanitize=thread and gcc 12, and running test-case
gdb.dwarf2/dwz.exp, we run into a data race between thread T2 and the main
thread in the same write:
...
Write of size 1 at 0x7b200000300c:^M
    #0 cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, \
    abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) gdb/dwarf2/read.c:6252 \
    (gdb+0x82f3b3)^M
...
which is here:
...
         this_cu->dwarf_version = cu->header.version;
...

Both writes are called from the parallel for in dwarf2_build_psymtabs_hard,
this one directly:
...
    #1 process_psymtab_comp_unit gdb/dwarf2/read.c:6774 (gdb+0x8304d7)^M
    #2 operator() gdb/dwarf2/read.c:7098 (gdb+0x8317be)^M
    #3 operator() gdbsupport/parallel-for.h:163 (gdb+0x872380)^M
...
and this via the PU import:
...
    #1 cooked_indexer::ensure_cu_exists(cutu_reader*, dwarf2_per_objfile*, \
    sect_offset, bool,  bool) gdb/dwarf2/read.c:17964 (gdb+0x85c43b)^M
    #2 cooked_indexer::index_imported_unit(cutu_reader*, unsigned char const*, \
    abbrev_info const*) gdb/dwarf2/read.c:18248 (gdb+0x85d8ff)^M
    #3 cooked_indexer::index_dies(cutu_reader*, unsigned char const*, \
    cooked_index_entry const*, bool) gdb/dwarf2/read.c:18302 (gdb+0x85dcdb)^M
    #4 cooked_indexer::make_index(cutu_reader*) gdb/dwarf2/read.c:18443 \
    (gdb+0x85e68a)^M
    #5 process_psymtab_comp_unit gdb/dwarf2/read.c:6812 (gdb+0x830879)^M
    #6 operator() gdb/dwarf2/read.c:7098 (gdb+0x8317be)^M
    #7 operator() gdbsupport/parallel-for.h:171 (gdb+0x8723e2)^M
...

Fix this by setting the field earlier, in read_comp_units_from_section.

The write in cutu_reader::cutu_reader() is still needed, in case
read_comp_units_from_section is not used.

Make the write conditional, such that it doesn't trigger if the field is
already set by read_comp_units_from_section.

Tested on x86_64-linux.
---
 gdb/dwarf2/read.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index d5088395fb1..67702d43aff 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -6249,7 +6249,11 @@ cutu_reader::cutu_reader (dwarf2_per_cu_data *this_cu,
 	    this_cu->length = cu->header.get_length ();
 	  else
 	    gdb_assert (this_cu->length == cu->header.get_length ());
-	  this_cu->dwarf_version = cu->header.version;
+	  /* Init this_cu->dwarf_version lazily.  */
+	  if (this_cu->dwarf_version == 0)
+	    this_cu->dwarf_version = cu->header.version;
+	  else
+	    gdb_assert (this_cu->dwarf_version == cu->header.version);
 	}
     }
 
@@ -7206,6 +7210,9 @@ read_comp_units_from_section (dwarf2_per_objfile *per_objfile,
       this_cu->length = cu_header.length + cu_header.initial_length_size;
       this_cu->is_dwz = is_dwz;
       this_cu->section = section;
+      /* Init this asap, to make sure the lazy init (which may be run in
+	 parallel for the cooked index case) isn't necessary.  */
+      this_cu->dwarf_version = cu_header.version;
 
       info_ptr = info_ptr + this_cu->length;
       per_objfile->per_bfd->all_comp_units.push_back (std::move (this_cu));
-- 
2.35.3