Re: [PATCH 16/36] rcu: Fix rcu_idle_exit()

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

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@xxxxxxxxxxxxx>
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
    Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

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);