linux-kernel - Re: [PATCH 3/3] x86/fpu: Remove init_task FPU state dependencies, add debugging warning

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Date: Wed, 5 Jun 2024 16:17:33 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave@...1.net>, Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH 3/3] x86/fpu: Remove init_task FPU state dependencies,
 add debugging warning

On 06/05, Ingo Molnar wrote:
>
> But the init task isn't supposed to be using the FPU in any case,

Afaics, the same is true for any PF_KTHREAD/USER_WORKER thread?

> +#ifdef CONFIG_X86_DEBUG_FPU
> +struct fpu *x86_task_fpu(struct task_struct *task)
> +{
> +	WARN_ON_ONCE(task == &init_task);
> +
> +	return (void *)task + sizeof(*task);
> +}

So perhaps WARN_ON_ONCE(task->flags & PF_KTHREAD) makes more sense?

Although in this case fpu_clone() can't use x86_task_fpu(dst) as I tried
to suggest in reply to 2/3.

This is offtopic right now, but I am wondering if it makes any sense to
make arch_task_struct_size depend on PF_KTHREAD... I mean, create another
task_struct_kthread_cachep used by alloc_task_struct_node() if PF_KTHREAD.

Oleg.