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

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Date:	Sun, 24 Aug 2014 13:05:39 -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 <suresh.b.siddha@...el.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()

I really dislike this one.

If I read it right, you now do *two* math_state_restore calls for each
FPU signal state restore. That's potentially quite expensive.

Also, you can actually end up with multiple threads pointing to the
same math state in init_task.thread.fpu.state, right? Why is that any
better than just having the save state temporarily contain garbage?

The other patches look sane, this one I really don't like. You may
have good reasons for it, but it's disgusting.

           Linus

On Sun, Aug 24, 2014 at 12:47 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> __restore_xstate_sig() calls math_state_restore() with preemption
> enabled, not good. But this is minor, [...]
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