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Date: Thu, 6 Jun 2024 14:00:38 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave@sr71.net>, Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH 3/3, v4] x86/fpu: Remove init_task FPU state
 dependencies, add debugging warning for PF_KTHREAD tasks
Message-ID: <20240606120038.GB22450@redhat.com>
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On 06/06, Ingo Molnar wrote:
>
> I changed the debug check to test for PF_KTHREAD, and to return NULL:
>
> +#ifdef CONFIG_X86_DEBUG_FPU
> +struct fpu *x86_task_fpu(struct task_struct *task)
> +{
> +	if (WARN_ON_ONCE(task->flags & PF_KTHREAD))
> +		return NULL;
> +
> +	return (void *)task + sizeof(*task);
> +}
> +#endif

How many users enable CONFIG_X86_DEBUG_FPU? Perhaps it makes sense
to check PF_KTHREAD unconditionally for the start, them add
if (IS_ENABLED(X86_DEBUG_FPU)). But I won't insist.


For the record, I think we can later change this code to check

	task->flags & (PF_KTHREAD | PF_USER_WORKER)

but I guess this needs some (simple) changes in the ptrace/coredump
paths.

Oleg.