Re: [PATCH v6 6/7] perf dso: Reference counting related fixes

[Ian Rogers <irogers@google.com>]

On Mon, May 6, 2024 at 7:44 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>
> On Sat, May 04, 2024 at 02:38:02PM -0700, Ian Rogers wrote:
> > Ensure gets and puts are better aligned fixing reference couting
> > checking problems.
> >
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> >  tools/perf/util/machine.c    |  4 +--
> >  tools/perf/util/map.c        |  1 +
> >  tools/perf/util/symbol-elf.c | 51 ++++++++++++++++++------------------
> >  3 files changed, 28 insertions(+), 28 deletions(-)
> >
> > diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> > index 0b8fb14f5ff6..4a4541a2e887 100644
> > --- a/tools/perf/util/machine.c
> > +++ b/tools/perf/util/machine.c
> > @@ -683,7 +683,7 @@ static int machine__process_ksymbol_register(struct machine *machine,
> >                                            struct perf_sample *sample __maybe_unused)
> >  {
> >       struct symbol *sym;
> > -     struct dso *dso;
> > +     struct dso *dso = NULL;
> >       struct map *map = maps__find(machine__kernel_maps(machine), event->ksymbol.addr);
> >       int err = 0;
> >
> > @@ -696,7 +696,6 @@ static int machine__process_ksymbol_register(struct machine *machine,
> >               }
> >               dso__set_kernel(dso, DSO_SPACE__KERNEL);
> >               map = map__new2(0, dso);
> > -             dso__put(dso);
> >               if (!map) {
> >                       err = -ENOMEM;
> >                       goto out;
> > @@ -735,6 +734,7 @@ static int machine__process_ksymbol_register(struct machine *machine,
> >       dso__insert_symbol(dso, sym);
> >  out:
> >       map__put(map);
> > +     dso__put(dso);
> >       return err;
> >  }
>
> This seems to match the patch description, good, just aligning the puts
>
> > diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
> > index 117c4bb78b35..e1d14936a60d 100644
> > --- a/tools/perf/util/map.c
> > +++ b/tools/perf/util/map.c
> > @@ -200,6 +200,7 @@ struct map *map__new(struct machine *machine, u64 start, u64 len,
> >                               dso__set_build_id(dso, dso__bid(header_bid_dso));
> >                               dso__set_header_build_id(dso, 1);
> >                       }
> > +                     dso__put(header_bid_dso);
> >               }
> >               dso__put(dso);
> >       }
>
> But this is a missing one, so not aligning, but fixing a separate issue,
> i.e. a missing put? Should go on a different patch, probably with a
> Fixes.

The Fixes would be for a patch in the series - albeit that we've spent
over half a year landing this and a month since that change landed:
https://lore.kernel.org/all/20240410064214.2755936-3-irogers@google.com/
The issue is that previously a find wouldn't do a get, so per the old
code no put is necessary. Gets were added to finds to cover the race
between a dso being returned from dsos and the item potentially being
removed from dsos.

> > diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> > index 3be5e8d1e278..e398abfd13a0 100644
> > --- a/tools/perf/util/symbol-elf.c
> > +++ b/tools/perf/util/symbol-elf.c
> > @@ -1419,7 +1419,7 @@ void __weak arch__sym_update(struct symbol *s __maybe_unused,
> >  static int dso__process_kernel_symbol(struct dso *dso, struct map *map,
> >                                     GElf_Sym *sym, GElf_Shdr *shdr,
> >                                     struct maps *kmaps, struct kmap *kmap,
> > -                                   struct dso **curr_dsop, struct map **curr_mapp,
> > +                                   struct dso **curr_dsop,
>
> This one removes an argument, so look like a separate patch as well,
> needs a description of why removing the argument is the right thing to
> do here.
>
> >                                     const char *section_name,
> >                                     bool adjust_kernel_syms, bool kmodule, bool *remap_kernel,
> >                                     u64 max_text_sh_offset)
> > @@ -1470,8 +1470,8 @@ static int dso__process_kernel_symbol(struct dso *dso, struct map *map,
> >                       map__set_pgoff(map, shdr->sh_offset);
> >               }
> >
> > -             *curr_mapp = map;
> > -             *curr_dsop = dso;
> > +             dso__put(*curr_dsop);
> > +             *curr_dsop = dso__get(dso);
> >               return 0;
> >       }
> >
> > @@ -1484,8 +1484,8 @@ static int dso__process_kernel_symbol(struct dso *dso, struct map *map,
> >        */
> >       if (kmodule && adjust_kernel_syms && is_exe_text(shdr->sh_flags) &&
> >           shdr->sh_offset <= max_text_sh_offset) {
> > -             *curr_mapp = map;
> > -             *curr_dsop = dso;
> > +             dso__put(*curr_dsop);
> > +             *curr_dsop = dso__get(dso);
> >               return 0;
> >       }
> >
> > @@ -1507,10 +1507,10 @@ static int dso__process_kernel_symbol(struct dso *dso, struct map *map,
> >               dso__set_binary_type(curr_dso, dso__binary_type(dso));
> >               dso__set_adjust_symbols(curr_dso, dso__adjust_symbols(dso));
> >               curr_map = map__new2(start, curr_dso);
> > -             dso__put(curr_dso);
> > -             if (curr_map == NULL)
> > +             if (curr_map == NULL) {
> > +                     dso__put(curr_dso);
> >                       return -1;
> > -
> > +             }
> >               if (dso__kernel(curr_dso))
> >                       map__kmap(curr_map)->kmaps = kmaps;
> >
> > @@ -1524,21 +1524,15 @@ static int dso__process_kernel_symbol(struct dso *dso, struct map *map,
> >               dso__set_symtab_type(curr_dso, dso__symtab_type(dso));
> >               if (maps__insert(kmaps, curr_map))
> >                       return -1;
> > -             /*
> > -              * Add it before we drop the reference to curr_map, i.e. while
> > -              * we still are sure to have a reference to this DSO via
> > -              * *curr_map->dso.
> > -              */
> >               dsos__add(&maps__machine(kmaps)->dsos, curr_dso);
> > -             /* kmaps already got it */
> > -             map__put(curr_map);
> >               dso__set_loaded(curr_dso);
> > -             *curr_mapp = curr_map;
> > +             dso__put(*curr_dsop);
> >               *curr_dsop = curr_dso;
> >       } else {
> > -             *curr_dsop = map__dso(curr_map);
> > -             map__put(curr_map);
> > +             dso__put(*curr_dsop);
> > +             *curr_dsop = dso__get(map__dso(curr_map));
> >       }
> > +     map__put(curr_map);
> >
> >       return 0;
> >  }
> > @@ -1549,11 +1543,9 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
> >  {
> >       struct kmap *kmap = dso__kernel(dso) ? map__kmap(map) : NULL;
> >       struct maps *kmaps = kmap ? map__kmaps(map) : NULL;
> > -     struct map *curr_map = map;
> > -     struct dso *curr_dso = dso;
> > +     struct dso *curr_dso = NULL;
> >       Elf_Data *symstrs, *secstrs, *secstrs_run, *secstrs_sym;
> >       uint32_t nr_syms;
> > -     int err = -1;
> >       uint32_t idx;
> >       GElf_Ehdr ehdr;
> >       GElf_Shdr shdr;
> > @@ -1656,6 +1648,7 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
> >       if (kmodule && adjust_kernel_syms)
> >               max_text_sh_offset = max_text_section(runtime_ss->elf, &runtime_ss->ehdr);
> >
> > +     curr_dso = dso__get(dso);
> >       elf_symtab__for_each_symbol(syms, nr_syms, idx, sym) {
> >               struct symbol *f;
> >               const char *elf_name = elf_sym__name(&sym, symstrs);
> > @@ -1744,9 +1737,13 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
> >                       --sym.st_value;
> >
> >               if (dso__kernel(dso)) {
> > -                     if (dso__process_kernel_symbol(dso, map, &sym, &shdr, kmaps, kmap, &curr_dso, &curr_map,
> > -                                                    section_name, adjust_kernel_syms, kmodule,
> > -                                                    &remap_kernel, max_text_sh_offset))
> > +                     if (dso__process_kernel_symbol(dso, map, &sym, &shdr,
> > +                                                    kmaps, kmap, &curr_dso,
> > +                                                    section_name,
> > +                                                    adjust_kernel_syms,
> > +                                                    kmodule,
> > +                                                    &remap_kernel,
> > +                                                    max_text_sh_offset))
>
> Can you please avoid reflowing to reduce the cost of reviewing? If this
> was just:
>
> -                       if (dso__process_kernel_symbol(dso, map, &sym, &shdr, kmaps, kmap, &curr_dso, &curr_map,
>                                                        section_name, adjust_kernel_syms, kmodule,
>                                                        &remap_kernel, max_text_sh_offset))
> +                       if (dso__process_kernel_symbol(dso, map, &sym, &shdr, kmaps, kmap, &curr_dso,
>                                                        section_name, adjust_kernel_syms, kmodule,
>                                                        &remap_kernel, max_text_sh_offset))
>
> The actual change would be quickly spotted as just the line where it
> takes place gets changed.
>
> I applied all the patches in this series but this one, please split it
> as described.

*sigh* 28 line change...

Ian

> The tests I run after each patch before this one passed, thanks for
> addressing the issues I reported,
>
> I'm pushing what I have to tmp.perf-tools-next,
>
> - Arnaldo
>
> >                               goto out_elf_end;
> >               } else if ((used_opd && runtime_ss->adjust_symbols) ||
> >                          (!used_opd && syms_ss->adjust_symbols)) {
> > @@ -1795,6 +1792,7 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
> >               __symbols__insert(dso__symbols(curr_dso), f, dso__kernel(dso));
> >               nr++;
> >       }
> > +     dso__put(curr_dso);
> >
> >       /*
> >        * For misannotated, zeroed, ASM function sizes.
> > @@ -1810,9 +1808,10 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
> >                       maps__fixup_end(kmaps);
> >               }
> >       }
> > -     err = nr;
> > +     return nr;
> >  out_elf_end:
> > -     return err;
> > +     dso__put(curr_dso);
> > +     return -1;
> >  }
> >
> >  int dso__load_sym(struct dso *dso, struct map *map, struct symsrc *syms_ss,
> > --
> > 2.45.0.rc1.225.g2a3ae87e7f-goog
> >