Re: [patch V4 part 1 36/36] rcu: Make RCU IRQ enter/exit functions rely on in_nmi()

Paul E. McKenney
2020-05-05T18:13:42Z

On Tue, May 05, 2020 at 03:16:38PM +0200, Thomas Gleixner wrote:
> From: Paul E. McKenney <paulmck@kernel.org>
>
> The rcu_nmi_enter_common() and rcu_nmi_exit_common() functions take an
> "irq" parameter that indicates whether these functions are invoked from
> an irq handler (irq==true) or an NMI handler (irq==false). However,
> recent changes have applied notrace to a few critical functions such
> that rcu_nmi_enter_common() and rcu_nmi_exit_common() many now rely
> on in_nmi(). Note that in_nmi() works no differently than before,
> but rather that tracing is now prohibited in code regions where in_nmi()
> would incorrectly report NMI state.
>
> Therefore remove the "irq" parameter and inlines rcu_nmi_enter_common() and
> rcu_nmi_exit_common() into rcu_nmi_enter() and rcu_nmi_exit(),
> respectively.

Not a bad job of ghostwriting, actually. I had forgotten about this
entirely, but did find my February 13th email containing this patch. ;-)

But why not make the commit log official?

------------------------------------------------------------------------

The rcu_nmi_enter_common() and rcu_nmi_exit_common() functions take an
"irq" parameter that indicates whether these functions have been invoked
from an irq handler (irq==true) or an NMI handler (irq==false). However,
recent changes have applied notrace to a number of critical functions,
thus allowing rcu_nmi_enter_common() and rcu_nmi_exit_common() to rely
on in_nmi(). Note that in_nmi() works no differently than before.
Instead, tracing is now prohibited in code regions where in_nmi() would
previously have incorrectly reported NMI state.

This commit therefore removes the "irq" parameter and inlines
rcu_nmi_enter_common() and rcu_nmi_exit_common() into rcu_nmi_enter()
and rcu_nmi_exit(), respectively.

------------------------------------------------------------------------

With that commit log, looks good to me!

Thanx, Paul

> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> kernel/rcu/tree.c | 47 +++++++++++++++--------------------------------
> 1 file changed, 15 insertions(+), 32 deletions(-)
>
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -627,16 +627,18 @@ noinstr void rcu_user_enter(void)
> }
> #endif /* CONFIG_NO_HZ_FULL */
>
> -/*
> +/**
> + * rcu_nmi_exit - inform RCU of exit from NMI context
> + *
> * If we are returning from the outermost NMI handler that interrupted an
> * RCU-idle period, update rdp->dynticks and rdp->dynticks_nmi_nesting
> * to let the RCU grace-period handling know that the CPU is back to
> * being RCU-idle.
> *
> - * If you add or remove a call to rcu_nmi_exit_common(), be sure to test
> + * If you add or remove a call to rcu_nmi_exit(), be sure to test
> * with CONFIG_RCU_EQS_DEBUG=y.
> */
> -static __always_inline void rcu_nmi_exit_common(bool irq)
> +noinstr void rcu_nmi_exit(void)
> {
> struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>
> @@ -667,7 +669,7 @@ static __always_inline void rcu_nmi_exit
> 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 (irq)
> + if (!in_nmi())
> rcu_prepare_for_idle();
> instr_end();
>
> @@ -675,22 +677,11 @@ static __always_inline void rcu_nmi_exit
> rcu_dynticks_eqs_enter();
> // ... but is no longer watching here.
>
> - if (irq)
> + if (!in_nmi())
> rcu_dynticks_task_enter();
> }
>
> /**
> - * rcu_nmi_exit - inform RCU of exit from NMI context
> - *
> - * If you add or remove a call to rcu_nmi_exit(), be sure to test
> - * with CONFIG_RCU_EQS_DEBUG=y.
> - */
> -void noinstr rcu_nmi_exit(void)
> -{
> - rcu_nmi_exit_common(false);
> -}
> -
> -/**
> * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
> *
> * Exit from an interrupt handler, which might possibly result in entering
> @@ -712,7 +703,7 @@ void noinstr rcu_nmi_exit(void)
> void noinstr rcu_irq_exit(void)
> {
> lockdep_assert_irqs_disabled();
> - rcu_nmi_exit_common(true);
> + rcu_nmi_exit();
> }
>
> /*
> @@ -801,7 +792,7 @@ void noinstr rcu_user_exit(void)
> #endif /* CONFIG_NO_HZ_FULL */
>
> /**
> - * rcu_nmi_enter_common - inform RCU of entry to NMI context
> + * rcu_nmi_enter - inform RCU of entry to NMI context
> * @irq: Is this call from rcu_irq_enter?
> *
> * If the CPU was idle from RCU's viewpoint, update rdp->dynticks and
> @@ -810,10 +801,10 @@ void noinstr rcu_user_exit(void)
> * long as the nesting level does not overflow an int. (You will probably
> * run out of stack space first.)
> *
> - * If you add or remove a call to rcu_nmi_enter_common(), be sure to test
> + * If you add or remove a call to rcu_nmi_enter(), be sure to test
> * with CONFIG_RCU_EQS_DEBUG=y.
> */
> -static __always_inline void rcu_nmi_enter_common(bool irq)
> +noinstr void rcu_nmi_enter(void)
> {
> long incby = 2;
> struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> @@ -831,18 +822,18 @@ static __always_inline void rcu_nmi_ente
> */
> if (rcu_dynticks_curr_cpu_in_eqs()) {
>
> - if (irq)
> + if (!in_nmi())
> rcu_dynticks_task_exit();
>
> // RCU is not watching here ...
> rcu_dynticks_eqs_exit();
> // ... but is watching here.
>
> - if (irq)
> + if (!in_nmi())
> rcu_cleanup_after_idle();
>
> incby = 1;
> - } else if (irq) {
> + } else if (!in_nmi()) {
> instr_begin();
> if (tick_nohz_full_cpu(rdp->cpu) &&
> rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
> @@ -877,14 +868,6 @@ static __always_inline void rcu_nmi_ente
> }
>
> /**
> - * rcu_nmi_enter - inform RCU of entry to NMI context
> - */
> -noinstr void rcu_nmi_enter(void)
> -{
> - rcu_nmi_enter_common(false);
> -}
> -
> -/**
> * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
> *
> * Enter an interrupt handler, which might possibly result in exiting
> @@ -909,7 +892,7 @@ noinstr void rcu_nmi_enter(void)
> noinstr void rcu_irq_enter(void)
> {
> lockdep_assert_irqs_disabled();
> - rcu_nmi_enter_common(true);
> + rcu_nmi_enter();
> }
>
> /*
>