Re: [PATCH rcu 10/11] srcu: Add FORCE_NEED_SRCU_NMI_SAFE Kconfig for testing

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Paul,

On Tue, 25 Mar 2025 at 15:36, Paul E. McKenney <paulmck@kernel.org> wrote:
> On Tue, Mar 25, 2025 at 09:04:31AM +0100, Geert Uytterhoeven wrote:
> > On Wed, 19 Feb 2025 at 16:44, Boqun Feng <boqun.feng@gmail.com> wrote:
> > > From: "Paul E. McKenney" <paulmck@kernel.org>
> > >
> > > The srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe() functions
> > > map to __srcu_read_lock() and __srcu_read_unlock() on systems like x86
> > > that have NMI-safe this_cpu_inc() operations.  This makes the underlying
> > > __srcu_read_lock_nmisafe() and __srcu_read_unlock_nmisafe() functions
> > > difficult to test on (for example) x86 systems, allowing bugs to creep in.
> > >
> > > This commit therefore creates a FORCE_NEED_SRCU_NMI_SAFE Kconfig that
> > > forces those underlying functions to be used even on systems where they
> > > are not needed, thus providing better testing coverage.
> > >
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> >
> > Thanks for your patch, which is now commit 536e8b9b80bc7a0a ("srcu:
> > Add FORCE_NEED_SRCU_NMI_SAFE Kconfig for testing") in linus/master
> >
> > > --- a/kernel/rcu/Kconfig
> > > +++ b/kernel/rcu/Kconfig
> > > @@ -65,6 +65,17 @@ config TREE_SRCU
> > >         help
> > >           This option selects the full-fledged version of SRCU.
> > >
> > > +config FORCE_NEED_SRCU_NMI_SAFE
> > > +       bool "Force selection of NEED_SRCU_NMI_SAFE"
> >
> > What am I supposed to answer here? "n" I guess.
> > What about distro and allmodconfig kernels?
>
> Yes, you should select "n" unless ...
>
> > > +       depends on !TINY_SRCU
> > > +       select NEED_SRCU_NMI_SAFE
> > > +       default n
> > > +       help
> > > +         This option forces selection of the NEED_SRCU_NMI_SAFE
> > > +         Kconfig option, allowing testing of srcu_read_lock_nmisafe()
> > > +         and srcu_read_unlock_nmisafe() on architectures (like x86)
> > > +         that select the ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.
> >
> > Perhaps this should depend on ARCH_HAS_NMI_SAFE_THIS_CPU_OPS?
>
> ... you are on a system selecting ARCH_HAS_NMI_SAFE_THIS_CPU_OPS and

So a dependency on ARCH_HAS_NMI_SAFE_THIS_CPU_OPS does make sense,
doesn't it?

> you would like to test the SRCU setup that needed only by systems that
> do not select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS.
>
> Ah.  I forgot to add "depends on RCU_EXPERT".

Yes, that makes sense.

> Apologies, I will fix this.  Does the patch show below do the trick?
>
>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit b5c8c6f89c6d7ac778e961ad4b883eada0c1f42a
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Tue Mar 25 07:31:45 2025 -0700
>
>     srcu: Make FORCE_NEED_SRCU_NMI_SAFE depend on RCU_EXPERT
>
>     The FORCE_NEED_SRCU_NMI_SAFE is useful only for those wishing to test
>     the SRCU code paths that accommodate architectures that do not have
>     NMI-safe per-CPU operations, that is, those architectures that do not
>     select the ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.  As such, this
>     is a specialized Kconfig option that is not intended for casual users.
>
>     This commit therefore hides it behind the RCU_EXPERT Kconfig option.
>
>     Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
>     Closes: https://lore.kernel.org/all/CAMuHMdX6dy9_tmpLkpcnGzxyRbe6qSWYukcPp=H1GzZdyd3qBQ@mail.gmail.com/
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index b3f985d41717a..cc4ce79f58aa6 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -68,6 +68,7 @@ config TREE_SRCU
>  config FORCE_NEED_SRCU_NMI_SAFE
>         bool "Force selection of NEED_SRCU_NMI_SAFE"
>         depends on !TINY_SRCU
> +       depends on RCU_EXPERT
>         select NEED_SRCU_NMI_SAFE
>         default n
>         help

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds