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

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Date:   Thu, 1 Sep 2022 16:18:23 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Zhen Lei <thunder.leizhen@...wei.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 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.

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
reduce the kernel binary size.

Best Regards,
Petr