linux-kernel - Re: [PATCH] livepatch: Move error print out of lock protection in klp_enable

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Message-ID: <c1f6c6ec-2681-8d7a-506a-4da72a963ce8@huawei.com>
Date:   Fri, 2 Sep 2022 09:28:59 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Petr Mladek <pmladek@...e.com>
CC:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        <live-patching@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] livepatch: Move error print out of lock protection in
 klp_enable_patch()



On 2022/9/1 22:18, Petr Mladek wrote:
> On Thu 2022-09-01 10:27:06, Zhen Lei wrote:
>> The patch->mod is not a protected object of mutex_lock(&klp_mutex). Since
>> it's in the error handling branch, it might not be helpful to reduce lock
>> conflicts, but it can reduce some code size.
>>
>> Before:
>>    text    data     bss     dec     hex filename
>>   10330     464       8   10802    2a32 kernel/livepatch/core.o
>>
>> After:
>>    text    data     bss     dec     hex filename
>>   10307     464       8   10779    2a1b kernel/livepatch/core.o
> 
> Please, is this part of some longterm effort to reduce the size of
> the kernel? Or is this just some random observation?
> 
> 
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> ---
>>  kernel/livepatch/core.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
>> index 42f7e716d56bf72..cb7abc821a50584 100644
>> --- a/kernel/livepatch/core.c
>> +++ b/kernel/livepatch/core.c
>> @@ -1041,9 +1041,9 @@ int klp_enable_patch(struct klp_patch *patch)
>>  	mutex_lock(&klp_mutex);
>>  
>>  	if (!klp_is_patch_compatible(patch)) {
>> +		mutex_unlock(&klp_mutex);
>>  		pr_err("Livepatch patch (%s) is not compatible with the already installed livepatches.\n",
>>  			patch->mod->name);
>> -		mutex_unlock(&klp_mutex);
> 
> I do not see how this change could reliably reduce the code size.
> 
> As Joe wrote, it looks like a random effect that is specific to a
> particular compiler version, compilation options, and architecture.
> 
> I am against these kind of random microptimizations. It is just a call
> for problems. If you move printk() outside of a lock, you need to make
> sure that the information is not racy.

OK.

	mutex_lock(&klp_mutex);
        if (!klp_is_patch_compatible(patch)) {
                mutex_unlock(&klp_mutex);
			<--------- Do you mean the incompatible patches maybe disabled at this point?
                pr_err("Livepatch patch (%s) ...\n", patch->mod->name);
                return -EINVAL;
        }

> 
> It might be safe in this particular case. But it is a bad practice.
> It adds an extra work. It is error-prone with questionable gain.
> 
> I am sorry but I NACK this patch. There must be better ways to

OK

> reduce the kernel binary size.
> 
> Best Regards,
> Petr
> .
> 

-- 
Regards,
  Zhen Lei