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

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Message-ID: <9dfa7d4f-6e3f-4084-a14f-beb1db06f817@paulmck-laptop>
Date: Sun, 7 Sep 2025 18:29:06 -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] srcu/tiny: Remove preempt_disable/enable() in
 srcu_gp_start_if_needed()

On Mon, Sep 08, 2025 at 08:31:55AM +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().
> 
> Fixes: 65b4a59557f6 ("srcu: Make Tiny SRCU explicitly disable preemption")
> Signed-off-by: Zqiang <qiang.zhang@...ux.dev>

Looks good, but what would be a good way to make this code defend itself
against being invoked from someplace else that did have preemption
enabled?  Especially given that the Tree SRCU version of this function
does get invoked with preemption enabled?

							Thanx, Paul

> ---
>  kernel/rcu/srcutiny.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index b52ec45698e8..417bd0e4457c 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -181,10 +181,8 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>  {
>  	unsigned long cookie;
>  
> -	preempt_disable();  // Needed for PREEMPT_LAZY
>  	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 +192,6 @@ 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
>