linux-kernel - Re: ptrace && fpu_lazy

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Date:	Sun, 15 Apr 2012 16:42:45 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <chuckebbert.lk@...il.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: ptrace && fpu_lazy_restore

On Sun, Apr 15, 2012 at 3:38 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> Suppose that fpu_owner_task exits on CPU_0, and then fork() reuses
> its task_struct. The new child is still fpu_owner_task and this is
> obviously wrong (unless of course another thread uses fpu).
>
> Initially I thought this should be fixed too, but it seems that
> "p->fpu_counter = 0" in copy_thread() saves us.
>
> This looks a bit fragile... And could you confirm this is really
> fine?

That one is done by design. That fpu_counter=0 in copy_thread() is
there explicitly to avoid the problem. Although it's possible that we
should reset last_cpu instead. However, that line was actually added
before the lazy thing - see commit cea20ca3f318.

> Btw, do we really need this "old->thread.fpu.last_cpu = ~0" in
> the "else" branch of switch_fpu_prepare()? Just curious, I guees
> this doesn't matter since we reset old->fpu_counter. But if we
> can remove this line, then perhaps we can another optimization.

Possibly not needed, but quite frankly, I'd rather have last_cpu never
contain some stale value.

                  Linus
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