Re: [PATCH 1/2] Pre-read .debug_aranges section

[Guinevere Larsen <blarsen@redhat.com>]

On 14/10/2023 22:56, Tom Tromey wrote:
> While working on background DWARF reading, I found a race case that I
> tracked down to the handling of the .debug_aranges section.  Currently
> the section data is only read in after the CUs have all been created.
> However, there's no real reason to do this -- it seems fine to read it
> a little earlier, when all the other necessary sections are read in.
>
> This patch makes this change, and updates the
> read_addrmap_from_aranges API to assert that the section is read in.

I tested this as well and see no issues either. My one comment is about 
describing the section as "read in".

It made it very confusing to read (I thought there was a word missing 
somewhere at first), and the description of the "readin" flag says:

     /* True if we have tried to read this section.  */

So I think it would be better to describe that assert as "assert that 
the section has been read" or some other way to make it explicit that 
"read" is the past tense.

With some small rewording around this bit, everything looks good to me, 
Reviewed-By: Guinevere Larsen <blarsen@redhat.com>

-- 
Cheers,
Guinevere Larsen
She/Her/Hers

>
> This patch slightly changes the read_addrmap_from_aranges API as well,
> to reject an empty section.  This seems better to me than what the
> current code does, which is try to read an empty section but then do
> no work.
>
> Regression tested on x86-64 Fedora 38.
> ---
>   gdb/dwarf2/read-debug-names.c |  1 +
>   gdb/dwarf2/read.c             | 11 ++++++-----
>   2 files changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/gdb/dwarf2/read-debug-names.c b/gdb/dwarf2/read-debug-names.c
> index 2e5067efb3d..89f5df6df90 100644
> --- a/gdb/dwarf2/read-debug-names.c
> +++ b/gdb/dwarf2/read-debug-names.c
> @@ -166,6 +166,7 @@ create_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
>   
>     addrmap_mutable mutable_map;
>   
> +  section->read (per_objfile->objfile);
>     if (read_addrmap_from_aranges (per_objfile, section, &mutable_map))
>       per_bfd->index_addrmap
>         = new (&per_bfd->obstack) addrmap_fixed (&per_bfd->obstack,
> diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
> index d4aec19d31d..13ac83395eb 100644
> --- a/gdb/dwarf2/read.c
> +++ b/gdb/dwarf2/read.c
> @@ -1586,6 +1586,7 @@ dwarf2_per_bfd::map_info_sections (struct objfile *objfile)
>     ranges.read (objfile);
>     rnglists.read (objfile);
>     addr.read (objfile);
> +  debug_aranges.read (objfile);
>   
>     for (auto &section : types)
>       section.read (objfile);
> @@ -1843,6 +1844,11 @@ read_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
>   			   dwarf2_section_info *section,
>   			   addrmap *mutable_map)
>   {
> +  /* Caller must ensure this is read in.  */
> +  gdb_assert (section->readin);
> +  if (section->empty ())
> +    return false;
> +
>     struct objfile *objfile = per_objfile->objfile;
>     bfd *abfd = objfile->obfd.get ();
>     struct gdbarch *gdbarch = objfile->arch ();
> @@ -1870,13 +1876,8 @@ read_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
>       }
>   
>     std::set<sect_offset> debug_info_offset_seen;
> -
> -  section->read (objfile);
> -
>     const bfd_endian dwarf5_byte_order = gdbarch_byte_order (gdbarch);
> -
>     const gdb_byte *addr = section->buffer;
> -
>     while (addr < section->buffer + section->size)
>       {
>         const gdb_byte *const entry_addr = addr;