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

Paul E. McKenney
2022-06-15T00:44:19Z

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);