Re: [PATCH RFC v2 rcu 3/8] srcu: Check for consistent per-CPU per-srcu_struct NMI safety

[Frederic Weisbecker <frederic@kernel.org>]

On Sun, Oct 02, 2022 at 04:51:03PM -0700, Paul E. McKenney wrote:
> On Mon, Oct 03, 2022 at 12:06:19AM +0200, Frederic Weisbecker wrote:
> > On Thu, Sep 29, 2022 at 11:07:26AM -0700, Paul E. McKenney wrote:
> > > This commit adds runtime checks to verify that a given srcu_struct uses
> > > consistent NMI-safe (or not) read-side primitives on a per-CPU basis.
> > > 
> > > Link: https://lore.kernel.org/all/20220910221947.171557773@linutronix.de/
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: John Ogness <john.ogness@linutronix.de>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > ---
> > >  include/linux/srcu.h     |  4 ++--
> > >  include/linux/srcutiny.h |  4 ++--
> > >  include/linux/srcutree.h |  9 +++++++--
> > >  kernel/rcu/srcutree.c    | 38 ++++++++++++++++++++++++++++++++------
> > >  4 files changed, 43 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > index 2cc8321c0c86..565f60d57484 100644
> > > --- a/include/linux/srcu.h
> > > +++ b/include/linux/srcu.h
> > > @@ -180,7 +180,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp
> > >  	int retval;
> > >  
> > >  	if (IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
> > > -		retval = __srcu_read_lock_nmisafe(ssp);
> > > +		retval = __srcu_read_lock_nmisafe(ssp, true);
> > >  	else
> > >  		retval = __srcu_read_lock(ssp);
> > 
> > Shouldn't it be checked also when CONFIG_NEED_SRCU_NMI_SAFE=n ?
> 
> You are asking why there is no "true" argument to __srcu_read_lock()?
> That is because it checks unconditionally.

It checks unconditionally but it always assumes not to be called as nmisafe.

For example on x86/arm64/loongarch, the same ssp used with both srcu_read_lock() and
srcu_read_lock_nmisafe() won't report an issue. But on powerpc it will.

My point is that strong archs should warn as well on behalf of others, to detect
mistakes early.

Thanks.