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

["Liao, Chang" <liaochang1@huawei.com>]

Hi, Mark and Will

Kindly ping.

I'd like to get your input on the next step. Should I send a v2 patch
that only removes the WARN_ON() as Will suggested? Or should I push
forward the refactoring of calling BRK hook [1].

Thanks.

[1] https://lore.kernel.org/all/20240906031930.746118-1-liaochang1@huawei.com/

在 2024/8/27 20:50, Mark Rutland 写道:
> 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@huawei.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
>>
>>
> 

-- 
BR
Liao, Chang