linux-kernel - Re: [PATCH v2] srcu/tiny: Remove preempt_disable/enable() in srcu_gp_start_if

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Message-ID: <4c7d977b-7d09-48e0-9a88-bae93fa5e401@paulmck-laptop>
Date: Wed, 10 Sep 2025 06:55:48 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zqiang <qiang.zhang@...ux.dev>
Cc: frederic@...nel.org, neeraj.upadhyay@...nel.org, joelagnelf@...dia.com,
	boqun.feng@...il.com, urezki@...il.com, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] srcu/tiny: Remove preempt_disable/enable() in
 srcu_gp_start_if_needed()

On Tue, Sep 09, 2025 at 09:39:28PM +0800, Zqiang wrote:
> Currently, the srcu_gp_start_if_needed() is always be invoked in
> preempt disable's critical section, this commit therefore remove
> redundant preempt_disable/enable() in srcu_gp_start_if_needed()
> and adds a call to lockdep_assert_preemption_disabled() in order
> to enable lockdep to diagnose mistaken invocations of this function
> from preempts-enabled code.
> 
> Fixes: 65b4a59557f6 ("srcu: Make Tiny SRCU explicitly disable preemption")
> Signed-off-by: Zqiang <qiang.zhang@...ux.dev>

Very good, applied for testing and further review, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/srcutiny.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index b52ec45698e8..b2da188133fc 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -181,10 +181,9 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>  {
>  	unsigned long cookie;
>  
> -	preempt_disable();  // Needed for PREEMPT_LAZY
> +	lockdep_assert_preemption_disabled();
>  	cookie = get_state_synchronize_srcu(ssp);
>  	if (ULONG_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie)) {
> -		preempt_enable();
>  		return;
>  	}
>  	WRITE_ONCE(ssp->srcu_idx_max, cookie);
> @@ -194,7 +193,6 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>  		else if (list_empty(&ssp->srcu_work.entry))
>  			list_add(&ssp->srcu_work.entry, &srcu_boot_list);
>  	}
> -	preempt_enable();
>  }
>  
>  /*
> -- 
> 2.48.1
>