linux-kernel - Re: ptrace && fpu_lazy

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Date:	Sun, 15 Apr 2012 16:46:48 -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 4:42 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> 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.

Put another way: I do think it would be a good idea to do the "reset
last_cpu" in copy_thread() too. It doesn't really cost us anything,
and it's cleaner to always just make sure that last_cpu is "valid"
(even if the fpu_owner_task is *also* used to invalidate it, and even
if we never use the lazy restore if fpu_counter is zero and thus
fpu.preload isn't set).

                    Linus
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