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 18:26:11 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>, 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>,
	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, Linus Torvalds wrote:
>
> On Wed, 5 Jun 2024 at 07:19, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > 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?
>
> I don't think so. We have various users of kernel_fpu_begin()/end()
> that are very much about things like crypto and RAID xor memory copies
> etc that will be used by kernel worker threads.

Yes, but kernel_fpu_begin() never does save_fpregs_to_fpstate() if
current->flags & PF_KTHREAD ?

Oleg.