linux-kernel - Re: [PATCH 2/5] x86, fpu: don't drop_fpu() in __restore_xstate_sig() if use_eager

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Message-ID: <CA+55aFwNUF+CLWKkDBe56xOtK_JTj9zHWip+_Ep41GJniN+sPw@mail.gmail.com>
Date:	Mon, 25 Aug 2014 09:27:37 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Bean Anderson <bean@...lsystems.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] x86, fpu: don't drop_fpu() in __restore_xstate_sig()
 if use_eager_fpu()

On Mon, Aug 25, 2014 at 7:41 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> I think this should be safe, because this thread and/or swapper/0 can
> do nothing with with fpu->state, and they should not use fpu.

.. but if that's the case, then what was wrong with the old code that
just copied the state over the unused space from the user space
buffer?

You can't have it both ways. Either the old code was fine (because it
doesn't use the buffer while it is in flux), or the new code is broken
(because it uses the shared buffer). Your choice.No?

                       Linus
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