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

[Tom de Vries]

On 6/29/22 17:29, Tom de Vries via Gdb-patches wrote:
> 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.
> 

I realized there's a similar write in the this_cu->is_debug_types == 
true clause in cutu_reader::cutu_reader(), which I missed.  So I 
introduced a member function set_version to make sure it's set in a 
consistent way.

Currently testing.

Thanks,
- Tom
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