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

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Jun 03, 2020 at 01:40:16PM +0200, Peter Zijlstra wrote:
> A KCSAN build revealed we have explicit annoations through atomic_*()
> usage, switch to arch_atomic_*() for the respective functions.
> 
> vmlinux.o: warning: objtool: rcu_nmi_exit()+0x4d: call to __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_dynticks_eqs_enter()+0x25: call to __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_nmi_enter()+0x4f: call to __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_dynticks_eqs_exit()+0x2a: call to __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: __rcu_is_watching()+0x25: call to __kcsan_check_access() leaves .noinstr.text section
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20200603084818.GB2627@hirez.programming.kicks-ass.net
> ---
>  kernel/rcu/tree.c |   11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> --- 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?

>  	// 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?

>  	// 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.

>  		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);

Also instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) as
follows:

o	rcu_nmi_exit(): After each following instrumentation_begin().

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().

o	rcu_nmi_enter(): After each following instrumentation_begin().

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().

>  }
>  
>  /*
> @@ -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?

							Thanx, Paul