Re: [PATCH 2/9] rcu: Fixup noinstr warnings

[Marco Elver]

On Thu, 4 Jun 2020 at 05:34, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Wed, Jun 03, 2020 at 07:13:20PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 03, 2020 at 09:46:00AM -0700, Paul E. McKenney wrote:
> >
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -250,7 +250,7 @@ static noinstr void rcu_dynticks_eqs_ent
> > > >    * next idle sojourn.
> > > >    */
> > > >   rcu_dynticks_task_trace_enter();  // Before ->dynticks update!
> > > > - seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
> > > > + seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
> > >
> > > To preserve KCSAN's ability to see this, there would be something like
> > > instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) prior
> > > to the instrumentation_end() invoked before rcu_dynticks_eqs_enter()
> > > in each of rcu_eqs_enter() and rcu_nmi_exit(), correct?
> >
> > Yes.
> >
> > > >   // RCU is no longer watching.  Better be in extended quiescent state!
> > > >   WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > >                (seq & RCU_DYNTICK_CTRL_CTR));
> > > > @@ -274,13 +274,13 @@ static noinstr void rcu_dynticks_eqs_exi
> > > >    * and we also must force ordering with the next RCU read-side
> > > >    * critical section.
> > > >    */
> > > > - seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
> > > > + seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
> > >
> > > And same here, but after the instrumentation_begin() following
> > > rcu_dynticks_eqs_exit() in both rcu_eqs_exit() and rcu_nmi_enter(),
> > > correct?
> >
> > Yep.
> >
> > > >   // RCU is now watching.  Better not be in an extended quiescent state!
> > > >   rcu_dynticks_task_trace_exit();  // After ->dynticks update!
> > > >   WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > >                !(seq & RCU_DYNTICK_CTRL_CTR));
> > > >   if (seq & RCU_DYNTICK_CTRL_MASK) {
> > > > -         atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
> > > > +         arch_atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
> > >
> > > This one is gone in -rcu.
> >
> > Good, because that would make things 'complicated' with the external
> > instrumentation call. And is actually the reason I didn't even attempt
> > it this time around.
> >
> > > >           smp_mb__after_atomic(); /* _exit after clearing mask. */
> > > >   }
> > > >  }
> > > > @@ -313,7 +313,7 @@ static __always_inline bool rcu_dynticks
> > > >  {
> > > >   struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > > >
> > > > - return !(atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
> > > > + return !(arch_atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
> >
> > The above is actually instrumented by KCSAN, due to arch_atomic_read()
> > being a READ_ONCE() and it now understanding volatile.
> >
> > > Also instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) as
>
> Right, this should instead be instrument_read(...).
>
> Though if KCSAN is unconditionally instrumenting volatile, how does
> this help?  Or does KCSAN's instrumentation of volatile somehow avoid
> causing trouble?

When used normally outside noinstr functions, because this is an
__always_inline function, it will be instrumented. Within noinstr
(which imply __no_kcsan) functions it should not be instrumented.

Thanks,
-- Marco


> > > follows:
> > >
> > > o   rcu_nmi_exit(): After each following instrumentation_begin().
> >
> > Yes
> >
> > > o   In theory in rcu_irq_exit_preempt(), but as this generates code
> > >     only in lockdep builds, it might not be worth worrying about.
> > >
> > > o   Ditto for rcu_irq_exit_check_preempt().
> > >
> > > o   Ditto for __rcu_irq_enter_check_tick().
> >
> > Not these, afaict they're all the above arch_atomic_read(), which is
> > instrumented due to volatile in these cases.
> >
> > > o   rcu_nmi_enter(): After each following instrumentation_begin().
> >
> > Yes
> >
> > > o   __rcu_is_watching() is itself noinstr:
> > >
> > >     o       idtentry_enter_cond_rcu(): After each following
> > >             instrumentation_begin().
> > >
> > > o   rcu_is_watching(): Either before or after the call to
> > >     rcu_dynticks_curr_cpu_in_eqs().
> >
> > Something like that yes.
> >
> > > >  }
> > > >
> > > >  /*
> > > > @@ -692,6 +692,7 @@ noinstr void rcu_nmi_exit(void)
> > > >  {
> > > >   struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > > >
> > > > + instrumentation_begin();
> > > >   /*
> > > >    * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
> > > >    * (We are exiting an NMI handler, so RCU better be paying attention
> > > > @@ -705,7 +706,6 @@ noinstr void rcu_nmi_exit(void)
> > > >    * leave it in non-RCU-idle state.
> > > >    */
> > > >   if (rdp->dynticks_nmi_nesting != 1) {
> > > > -         instrumentation_begin();
> > > >           trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2,
> > > >                             atomic_read(&rdp->dynticks));
> > > >           WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
> > > > @@ -714,7 +714,6 @@ noinstr void rcu_nmi_exit(void)
> > > >           return;
> > > >   }
> > > >
> > > > - instrumentation_begin();
> > > >   /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> > > >   trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
> > > >   WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
> > >
> > > This one looks to be having no effect on instrumentation of atomics, but
> > > rather coalescing a pair of instrumentation_begin() into one.
> > >
> > > Do I understand correctly?
> >
> > Almost, it puts the WARN_ON_ONCE()s under instrumentation_begin() too,
> > and that makes a differnce, iirc it was the
> > rcu_dynticks_curr_cpu_in_eqs() call that stood out. But that could've
> > been before I switched it to arch_atomic_read(). In any case, I find
> > this form a lot clearer.
>
> Got it, thank you.
>
>                                                 Thanx, Paul