linux-kernel - Re: [PATCH] arm64: Return early when break handler is found on linked-list

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Message-ID: <Zs3LnYkXL5sg2yBH@J2N7QTR9R3.cambridge.arm.com>
Date: Tue, 27 Aug 2024 13:50:37 +0100
From: Mark Rutland <mark.rutland@....com>
To: Liao Chang <liaochang1@...wei.com>
Cc: catalin.marinas@....com, will@...nel.org, ptosi@...gle.com,
	oliver.upton@...ux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: Return early when break handler is found on
 linked-list

On Tue, Aug 27, 2024 at 11:00:46AM +0000, Liao Chang wrote:
> The search for breakpoint handlers iterate through the entire
> linked list. Given that all registered hook has a valid fn field, and no
> registered hooks share the same mask and imm. This commit optimize the
> efficiency slightly by returning early as a matching handler is found.
> 
> Signed-off-by: Liao Chang <liaochang1@...wei.com>

This looks fine, though I'd love if we could clean this up to remove the
linked list entirely by separating the user/kernel entrypoints and using
a switch statement to decide the handler based on the immediate. That'd
also remove the need for RCU protection.

Last I looked that would require some largely mechanical restructuring,
and the only painful bit was the hooks that KGDB uses, since those are
the only ones that actually get unregistered.

Mark.

> ---
>  arch/arm64/kernel/debug-monitors.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index 024a7b245056..fc998956f44c 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -281,6 +281,7 @@ static LIST_HEAD(kernel_break_hook);
>  
>  void register_user_break_hook(struct break_hook *hook)
>  {
> +	WARN_ON(!hook->fn);
>  	register_debug_hook(&hook->node, &user_break_hook);
>  }
>  
> @@ -291,6 +292,7 @@ void unregister_user_break_hook(struct break_hook *hook)
>  
>  void register_kernel_break_hook(struct break_hook *hook)
>  {
> +	WARN_ON(!hook->fn);
>  	register_debug_hook(&hook->node, &kernel_break_hook);
>  }
>  
> @@ -303,7 +305,6 @@ static int call_break_hook(struct pt_regs *regs, unsigned long esr)
>  {
>  	struct break_hook *hook;
>  	struct list_head *list;
> -	int (*fn)(struct pt_regs *regs, unsigned long esr) = NULL;
>  
>  	list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
>  
> @@ -313,10 +314,10 @@ static int call_break_hook(struct pt_regs *regs, unsigned long esr)
>  	 */
>  	list_for_each_entry_rcu(hook, list, node) {
>  		if ((esr_brk_comment(esr) & ~hook->mask) == hook->imm)
> -			fn = hook->fn;
> +			return hook->fn(regs, esr);
>  	}
>  
> -	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
> +	return DBG_HOOK_ERROR;
>  }
>  NOKPROBE_SYMBOL(call_break_hook);
>  
> -- 
> 2.34.1
> 
>