Subject: Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 17 Jul 2019 23:22:15 -0700

On Thu, Jul 18, 2019 at 02:43:58PM +0900, Masami Hiramatsu wrote:
> Remove rcu_read_lock()/rcu_read_unlock() from debug exception
> handlers since the software breakpoint can be hit on idle task.

The exception entry and exit use irq_enter() and irq_exit(), in this
case, correct?  Otherwise RCU will be ignoring this CPU.

							Thanx, Paul

> Actually, we don't need it because those handlers run in exception
> context where the interrupts are disabled. This means those are never
> preempted.
> 
> Reported-by: Naresh Kamboju <[email-masked]>
> Cc: Paul E. McKenney <[email-masked]>
> Signed-off-by: Masami Hiramatsu <[email-masked]>
> ---
>  arch/arm64/kernel/debug-monitors.c |   14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index f8719bd30850..48222a4760c2 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -207,16 +207,16 @@ static int call_step_hook(struct pt_regs *regs, unsigned int esr)
>  
>  	list = user_mode(regs) ? &user_step_hook : &kernel_step_hook;
>  
> -	rcu_read_lock();
> -
> +	/*
> +	 * Since single-step exception disables interrupt, this function is
> +	 * entirely not preemptible, and we can use rcu list safely here.
> +	 */
>  	list_for_each_entry_rcu(hook, list, node)	{
>  		retval = hook->fn(regs, esr);
>  		if (retval == DBG_HOOK_HANDLED)
>  			break;
>  	}
>  
> -	rcu_read_unlock();
> -
>  	return retval;
>  }
>  NOKPROBE_SYMBOL(call_step_hook);
> @@ -305,14 +305,16 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
>  
>  	list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
>  
> -	rcu_read_lock();
> +	/*
> +	 * Since brk exception disables interrupt, this function is
> +	 * entirely not preemptible, and we can use rcu list safely here.
> +	 */
>  	list_for_each_entry_rcu(hook, list, node) {
>  		unsigned int comment = esr & ESR_ELx_BRK64_ISS_COMMENT_MASK;
>  
>  		if ((comment & ~hook->mask) == hook->imm)
>  			fn = hook->fn;
>  	}
> -	rcu_read_unlock();
>  
>  	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
>  }
> 

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