Current rcu_idle_exit() is terminally broken because it uses
local_irq_{save,restore}(), which are traced which uses RCU.

However, now that all the callers are sure to have IRQs disabled, we
can remove these calls.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -659,7 +659,7 @@ static noinstr void rcu_eqs_enter(bool u
  * If you add or remove a call to rcu_idle_enter(), be sure to test with
  * CONFIG_RCU_EQS_DEBUG=y.
  */
-void rcu_idle_enter(void)
+void noinstr rcu_idle_enter(void)
 {
 	lockdep_assert_irqs_disabled();
 	rcu_eqs_enter(false);
@@ -896,13 +896,10 @@ static void noinstr rcu_eqs_exit(bool us
  * If you add or remove a call to rcu_idle_exit(), be sure to test with
  * CONFIG_RCU_EQS_DEBUG=y.
  */
-void rcu_idle_exit(void)
+void noinstr rcu_idle_exit(void)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
+	lockdep_assert_irqs_disabled();
 	rcu_eqs_exit(false);
-	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_exit);

On Wed, Jun 08, 2022 at 04:27:39PM +0200, Peter Zijlstra wrote:
> Current rcu_idle_exit() is terminally broken because it uses
> local_irq_{save,restore}(), which are traced which uses RCU.
> 
> However, now that all the callers are sure to have IRQs disabled, we
> can remove these calls.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Paul E. McKenney <paulmck@kernel.org>

We have some fun conflicts between this series and Frederic's context-tracking
series.  But it looks like these can be resolved by:

1.	A patch on top of Frederic's series that provides the old rcu_*()
	names for the functions now prefixed with ct_*() such as
	ct_idle_exit().

2.	Another patch on top of Frederic's series that takes the
	changes remaining from this patch, shown below.  Frederic's
	series uses raw_local_irq_save() and raw_local_irq_restore(),
	which can then be removed.

Or is there a better way to do this?

							Thanx, Paul

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

commit f64cee8c159e9863a74594efe3d33fb513a6a7b5
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Tue Jun 14 17:24:43 2022 -0700

    context_tracking: Interrupts always disabled for ct_idle_exit()
    
    Now that the idle-loop cleanups have ensured that rcu_idle_exit() is
    always invoked with interrupts disabled, remove the interrupt disabling
    in favor of a debug check.
    
    Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    Cc: Frederic Weisbecker <frederic@kernel.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 1da44803fd319..99310cf5b0254 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -332,11 +332,8 @@ EXPORT_SYMBOL_GPL(ct_idle_enter);
  */
 void noinstr ct_idle_exit(void)
 {
-	unsigned long flags;
-
-	raw_local_irq_save(flags);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
 	ct_kernel_enter(false, RCU_DYNTICKS_IDX - CONTEXT_IDLE);
-	raw_local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(ct_idle_exit);

On Wed, Jun 08, 2022 at 04:27:39PM +0200, Peter Zijlstra wrote:
> Current rcu_idle_exit() is terminally broken because it uses
> local_irq_{save,restore}(), which are traced which uses RCU.
> 
> However, now that all the callers are sure to have IRQs disabled, we
> can remove these calls.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Paul E. McKenney <paulmck@kernel.org>

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  kernel/rcu/tree.c |    9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -659,7 +659,7 @@ static noinstr void rcu_eqs_enter(bool u
>   * If you add or remove a call to rcu_idle_enter(), be sure to test with
>   * CONFIG_RCU_EQS_DEBUG=y.
>   */
> -void rcu_idle_enter(void)
> +void noinstr rcu_idle_enter(void)
>  {
>  	lockdep_assert_irqs_disabled();
>  	rcu_eqs_enter(false);
> @@ -896,13 +896,10 @@ static void noinstr rcu_eqs_exit(bool us
>   * If you add or remove a call to rcu_idle_exit(), be sure to test with
>   * CONFIG_RCU_EQS_DEBUG=y.
>   */
> -void rcu_idle_exit(void)
> +void noinstr rcu_idle_exit(void)
>  {
> -	unsigned long flags;
> -
> -	local_irq_save(flags);
> +	lockdep_assert_irqs_disabled();
>  	rcu_eqs_exit(false);
> -	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_idle_exit);
>  
> 
>