linux-kernel - Re: [PATCH 1/3] coredump: flush the fpu exit state for proper multi-threaded core dump

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Date:	Thu, 10 May 2012 10:04:56 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>, hpa@...or.com,
	mingo@...e.hu, linux-kernel@...r.kernel.org,
	suresh@...stanetworks.com
Subject: Re: [PATCH 1/3] coredump: flush the fpu exit state for proper
 multi-threaded core dump

On Thu, May 10, 2012 at 9:55 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> My point was, there is no any guarantee prepare_to_copy() does the flush.
> An architecture can do this in copy_thread() or arch_dup_task_struct(),
> for example. In fact I do not understand why x86 doesn't do this.

I agree that it would actually make more sense to do in
arch_dup_task_struct(). I had trouble finding where the heck the
fork() code did the FPU fixes back when I was fighting the FPU
corruption thing.

The prepare_to_copy() thing is, I think, purely historical, and I
think we should in fact get rid of it. Everybody else makes it a
no-op, I think, with a *few* exceptions that seem to have copied the
x86 model of flushing the FPU there.

So if somebody sends me a patch to remove that thing, and move the few
existing users to arch_dup_task_struct(), I'd take it.

I think it would be a mistake to use it in the exit path. Make an
explicit "drop_thread_state()" or similar macro, which can undo FPU
state and possibly other architecture state.

                       Linus
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