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

[Paul E. McKenney]

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@xxxxxxxxxxxxx/
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: John Ogness <john.ogness@xxxxxxxxxxxxx>
> > Cc: Petr Mladek <pmladek@xxxxxxxx>
> > ---
> >  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.  OK, so why the
"true" argument to __srcu_read_lock_nmisafe(), you ask?  Because
srcu_gp_start_if_needed() needs to call __srcu_read_lock_nmisafe()
while suppressing the checking, which it does by passing in "false".
In turn because srcu_gp_start_if_needed() cannot always tell whether
its srcu_struct is or is not NMI-safe.

							Thanx, Paul