Re: [PATCH v19 34/40] rcu/update: fix same dept key collision between various types of RCU

[Byungchul Park]

On Tue, Jul 07, 2026 at 07:33:52AM +0000, sashiko-bot@xxxxxxxxxx wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Low] Unused struct member `dkey` inflates stack footprint
> --
> 
> commit de6aa9797881496b4a8d7d66626b1bfd19cfdd54
> Author: Yunseong Kim <ysk@xxxxxxxxxxx>
> 
> rcu/update: fix same dept key collision between various types of RCU
> 
> This commit addresses a false positive circular dependency report in dept
> by assigning separate dept keys and maps to each struct rcu_synchronize
> execution context.
> 
> > diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
> > --- a/include/linux/rcupdate_wait.h
> > +++ b/include/linux/rcupdate_wait.h
> > @@ -19,17 +19,20 @@ struct rcu_synchronize {
> >
> >       /* This is for debugging. */
> >       struct rcu_gp_oldstate oldstate;
> > +     struct dept_map dmap;
> > +     struct dept_key dkey;
> 
> [Severity: Low]
> Does this code ever use the dkey member of struct rcu_synchronize?
> 
> The _wait_rcu_gp() macro defines its own static struct dept_key __key,
> which it passes as an argument to __wait_rcu_gp(). This leaves the dkey
> member of struct rcu_synchronize completely unused and uninitialized, while
> needlessly increasing the stack footprint of the rs_array allocated in the
> macro.

I will fix it.

	Byungchul

> >  };
> >  void wakeme_after_rcu(struct rcu_head *head);
> >
> >  void __wait_rcu_gp(bool checktiny, unsigned int state, int n, call_rcu_func_t *crcu_array,
> > -                struct rcu_synchronize *rs_array);
> > +                struct rcu_synchronize *rs_array, struct dept_key *dkey);
> >
> >  #define _wait_rcu_gp(checktiny, state, ...) \
> > -do {                                                                                         \
> > -     call_rcu_func_t __crcu_array[] = { __VA_ARGS__ };                                       \
> > -     struct rcu_synchronize __rs_array[ARRAY_SIZE(__crcu_array)];                            \
> > -     __wait_rcu_gp(checktiny, state, ARRAY_SIZE(__crcu_array), __crcu_array, __rs_array);    \
> > +do {                                                                                                 \
> > +     call_rcu_func_t __crcu_array[] = { __VA_ARGS__ };                                               \
> > +     static struct dept_key __key;                                                                   \
> > +     struct rcu_synchronize __rs_array[ARRAY_SIZE(__crcu_array)];                                    \
> > +     __wait_rcu_gp(checktiny, state, ARRAY_SIZE(__crcu_array), __crcu_array, __rs_array, &__key);    \
> >  } while (0)
> 
> [ ... ]
> 
> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> 
> [ ... ]
> 
> > @@ -426,7 +426,8 @@ void __wait_rcu_gp(bool checktiny, unsigned int state, int n, call_rcu_func_t *c
> >                               break;
> >               if (j == i) {
> >                       init_rcu_head_on_stack(&rs_array[i].head);
> > -                     init_completion(&rs_array[i].completion);
> > +                     sdt_map_init_key(&rs_array[i].dmap, dkey);
> > +                     init_completion_dmap(&rs_array[i].completion, &rs_array[i].dmap);
> >                       (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
> >               }
> >       }
> 
> --
> Sashiko AI review · https://sashiko.dev/#/patchset/20260706061928.66713-1-byungchul@xxxxxx?part=34