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

[Paul E. McKenney]

On Mon, Jun 15, 2020 at 06:24:27PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 15, 2020 at 05:55:13PM +0200, Peter Zijlstra wrote:
> > On Mon, Jun 15, 2020 at 05:49:05PM +0200, Peter Zijlstra wrote:
> > > @@ -983,13 +993,17 @@ noinstr void rcu_nmi_enter(void)
> > >  		if (!in_nmi())
> > >  			rcu_cleanup_after_idle();
> > >  
> > > +		instrumentation_begin();
> > > +		// instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
> > > +		instrument_atomic_read(&rdp->dynticks, sizeof(rdp->dynticks));
> > > +		// instrumentation for the noinstr rcu_dynticks_eqs_exit()
> > > +		instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> > > +
> > >  		incby = 1;
> > >  	} else if (!in_nmi()) {
> > >  		instrumentation_begin();
> > >  		rcu_irq_enter_check_tick();
> > > -		instrumentation_end();
> > >  	}
> > > -	instrumentation_begin();
> > >  	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
> > >  			  rdp->dynticks_nmi_nesting,
> > >  			  rdp->dynticks_nmi_nesting + incby, atomic_read(&rdp->dynticks));
> > 
> > Oh, that's lost a possible instrumentation_begin() :/ But weirdly
> > objtool didn't complain about that... Let me poke at that.

This merge window has been quite the trainwreck, hasn't it?  :-/

> Like so then...

Looks plausible, firing up some tests.

							Thanx, Paul

> ---
> Subject: rcu: Fixup noinstr warnings
> 
> 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
> 
> Additionally, without the NOP in instrumentation_begin(), objtool would
> not detect the lack of the 'else instrumentation_begin();' branch in
> rcu_nmi_enter().
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  include/linux/compiler.h |    2 +-
>  kernel/rcu/tree.c        |   33 +++++++++++++++++++++++++--------
>  2 files changed, 26 insertions(+), 9 deletions(-)
> 
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -123,7 +123,7 @@ void ftrace_likely_update(struct ftrace_
>  #ifdef CONFIG_DEBUG_ENTRY
>  /* Begin/end of an instrumentation safe region */
>  #define instrumentation_begin() ({					\
> -	asm volatile("%c0:\n\t"						\
> +	asm volatile("%c0: nop\n\t"						\
>  		     ".pushsection .discard.instr_begin\n\t"		\
>  		     ".long %c0b - .\n\t"				\
>  		     ".popsection\n\t" : : "i" (__COUNTER__));		\
> --- 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);
>  	// 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);
>  	// 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);
>  		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);
>  }
>  
>  /*
> @@ -633,6 +633,10 @@ static noinstr void rcu_eqs_enter(bool u
>  	do_nocb_deferred_wakeup(rdp);
>  	rcu_prepare_for_idle();
>  	rcu_preempt_deferred_qs(current);
> +
> +	// instrumentation for the noinstr rcu_dynticks_eqs_enter()
> +	instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> +
>  	instrumentation_end();
>  	WRITE_ONCE(rdp->dynticks_nesting, 0); /* Avoid irq-access tearing. */
>  	// RCU is watching here ...
> @@ -692,6 +696,7 @@ 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 +710,6 @@ 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. */
> @@ -714,13 +718,15 @@ 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. */
>  
>  	if (!in_nmi())
>  		rcu_prepare_for_idle();
> +
> +	// instrumentation for the noinstr rcu_dynticks_eqs_enter()
> +	instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
>  	instrumentation_end();
>  
>  	// RCU is watching here ...
> @@ -838,6 +844,10 @@ static void noinstr rcu_eqs_exit(bool us
>  	rcu_dynticks_eqs_exit();
>  	// ... but is watching here.
>  	instrumentation_begin();
> +
> +	// instrumentation for the noinstr rcu_dynticks_eqs_exit()
> +	instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> +
>  	rcu_cleanup_after_idle();
>  	trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
>  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> @@ -983,13 +993,20 @@ noinstr void rcu_nmi_enter(void)
>  		if (!in_nmi())
>  			rcu_cleanup_after_idle();
>  
> +		instrumentation_begin();
> +		// instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
> +		instrument_atomic_read(&rdp->dynticks, sizeof(rdp->dynticks));
> +		// instrumentation for the noinstr rcu_dynticks_eqs_exit()
> +		instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> +
>  		incby = 1;
>  	} else if (!in_nmi()) {
>  		instrumentation_begin();
>  		rcu_irq_enter_check_tick();
> -		instrumentation_end();
> +	} else {
> +		instrumentation_begin();
>  	}
> -	instrumentation_begin();
> +
>  	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
>  			  rdp->dynticks_nmi_nesting,
>  			  rdp->dynticks_nmi_nesting + incby, atomic_read(&rdp->dynticks));