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

[Peter Zijlstra]

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
> 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.