Re: [PATCH v2] rcu: Remove local_irq_save/restore() in rcu_preempt_deferred_qs_handler()

["Zqiang" <qiang.zhang@linux.dev>]

> 
> On Wed, Aug 13, 2025 at 09:30:02PM +0800, Zqiang wrote:
> 
> > 
> > Currently, the per-cpu rcu_data structure's->defer_qs_iw is initialized by
> >  IRQ_WORK_INIT_HARD(), this means the rcu_preempt_deferred_qs_handler()
> >  always be executed in the hardirq context of irq-disabled.
> >  This commit therefore remove local_irq_save/restore() operations in
> >  rcu_preempt_deferred_qs_handler() and add lockdep_assert_irqs_disabled()
> >  check, if someone mistakenly invokes this function in hardirq enabled
> >  context will splat.
> >  
> >  Signed-off-by: Zqiang <qiang.zhang@linux.dev>
> > 
> Queued for further review and testing. With luck, this will make the
> v6.18 merge window. I could not resist editing the commit log, so could
> you please check whether I messed something up?

No problem, thanks for editing the commit log :) .

Thanks
Zqiang

> 
>  Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 523e71ba3007f8f7c260b9c2baf69c9461a84f55
> Author: Zqiang <qiang.zhang@linux.dev>
> Date: Wed Aug 13 21:30:02 2025 +0800
> 
>  rcu: Remove local_irq_save/restore() in rcu_preempt_deferred_qs_handler()
>  
>  The per-CPU rcu_data structure's ->defer_qs_iw field is initialized
>  by IRQ_WORK_INIT_HARD(), which means that the subsequent invocation of
>  rcu_preempt_deferred_qs_handler() will always be executed with interrupts
>  disabled. This commit therefore removes the local_irq_save/restore()
>  operations from rcu_preempt_deferred_qs_handler() and adds a call to
>  lockdep_assert_irqs_disabled() in order to enable lockdep to diagnose
>  mistaken invocations of this function from interrupts-enabled code.
>  
>  Signed-off-by: Zqiang <qiang.zhang@linux.dev>
>  Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index fc14adf15cbb34..57e2ae51c0b2cb 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -626,11 +626,10 @@ notrace void rcu_preempt_deferred_qs(struct task_struct *t)
>  */
>  static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
>  {
> - unsigned long flags;
>  struct rcu_data *rdp;
>  
> + lockdep_assert_irqs_disabled();
>  rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
> - local_irq_save(flags);
>  
>  /*
>  * If the IRQ work handler happens to run in the middle of RCU read-side
> @@ -647,8 +646,6 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
>  */
>  if (rcu_preempt_depth() > 0)
>  WRITE_ONCE(rdp->defer_qs_iw_pending, DEFER_QS_IDLE);
> -
> - local_irq_restore(flags);
>  }
>  
>  /*
>