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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Sat, 8 Jun 2024 09:26:36 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: 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>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH 2/3] x86/fpu: Remove the thread::fpu pointer
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Ingo Molnar <mingo@kernel.org> wrote:
> 
> > 
> > * Oleg Nesterov <oleg@redhat.com> wrote:
> > 
> > > On 06/05, Ingo Molnar wrote:
> > > >
> > > > @@ -591,13 +591,11 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
> > > >  	 * This is safe because task_struct size is a multiple of cacheline size.
> > > >  	 */
> > > >  	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
> > > > -	struct fpu *src_fpu = current->thread.fpu;
> > > > +	struct fpu *src_fpu = x86_task_fpu(current);
> > > 
> > > I think this patch can also change
> > > 
> > > 	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
> > > 
> > > above to use x86_task_fpu(dst).
> > 
> > Yeah, so I'd prefer to keep it open coded, because of the comment and the 
> > debug check makes a lot more sense if the pointer calculation is visible:
> 
> On a second thought I changed it to your suggested variant:
> 
>         struct fpu *src_fpu = x86_task_fpu(current);
>         struct fpu *dst_fpu = x86_task_fpu(dst);
> 
> because you are right, it's in fact easier to read this way.

On a third thought, while more readable, this doesn't work in practice with 
the current scheme, because x86_task_fpu() gets called on kthreads in 
fpu_clone(), which trips up the new debugging code.

We could resolve it by special-casing PF_KTHREAD here too, but that weakens 
the whole readability argument. I'll leave it as-is for now.

Thanks,

	Ingo