linux-kernel - Re: [PATCH] arm64: remove unneeded BUILD_BUG

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Date: Tue, 6 Feb 2024 09:39:30 +0800
From: Dawei Li <dawei.li@...ngroup.cn>
To: Mark Rutland <mark.rutland@....com>
Cc: catalin.marinas@....com, will@...nel.org, mcgrof@...nel.org,
	jpoimboe@...nel.org, j.granados@...sung.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	set_pte_at@...look.com
Subject: Re: [PATCH] arm64: remove unneeded BUILD_BUG_ON assertion

Hi Mark,

Thanks for reviewing.

On Mon, Feb 05, 2024 at 12:06:18PM +0000, Mark Rutland wrote:
> On Fri, Feb 02, 2024 at 12:02:11PM +0800, Dawei Li wrote:
> > Since commit c02433dd6de3 ("arm64: split thread_info from task stack"),
> > CONFIG_THREAD_INFO_IN_TASK is enabled unconditionally for arm64. So
> > remove this always-true assertion from arch_dup_task_struct.
> > 
> > Signed-off-by: Dawei Li <dawei.li@...ngroup.cn>
> > ---
> >  arch/arm64/kernel/process.c | 3 ---
> >  1 file changed, 3 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 7387b68c745b..4ae31b7af6c3 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -290,9 +290,6 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> >  		fpsimd_preserve_current_state();
> >  	*dst = *src;
> >  
> > -	/* We rely on the above assignment to initialize dst's thread_flags: */
> > -	BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));
> > -
> 
> Does the above cause any problem today, or is this patch just a cleanup?

It's just a cleanup patch.

> 
> For the benefit of other reviewers, the assertion and comment were added in
> commit:
> 
>   4585fc59c0e8 ("arm64/sve: Fix wrong free for task->thread.sve_state")
> 
> .. back in 2019, 3 years after commit:
> 
>   c02433dd6de3 ("arm64: split thread_info from task stack")
> 
> The comment and assertion were a safety-net for backports, since commit
> 4585fc59c0e8 was a fix which dependend upon the thread_info being contained
> within task_struct, and couldn't be backported to kernels without
> CONFIG_THREAD_INFO_IN_TASK.

Thanks for the update! It helps.

> 
> I'm sure that we currently have plenty of other code with a similar (but
> undocumented) dependency. Given we've unconditionally selected

Totally agreed.

> CONFIG_THREAD_INFO_IN_TASK since v4.10, and the oldest longterm stable kernel
> is v4.19 (with v4.14 having EOL'd last month), I think it makes sense to delete
> the assertion and comment.
> 
> So FWIW:
> 
> Acked-by: Mark Rutland <mark.rutland@....com>
> 
> Mark.
> 

Thanks,

    Dawei