linux-kernel - Re: [PATCH 3/4] x86/fpu: use restore_init_xstate() instead of math_state

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Message-ID: <20150313155121.GH31998@pd.tnic>
Date:	Fri, 13 Mar 2015 16:51:21 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 3/4] x86/fpu: use restore_init_xstate() instead of
 math_state_restore() on kthread exec

On Fri, Mar 13, 2015 at 03:45:14PM +0100, Oleg Nesterov wrote:
> Because in math_state_restore() case kernel_fpu_begin()->__save_init_fpu()
> will overwrite (corrupt) the same fpu->state buffer we need to restore.
> Without kernel_fpu_disable().

Yes.

> restore_init_xstate() obviously differs because it reads init_xstate_buf,
> we do not care at all if kernel_fpu_begin() in between overwrites ->state.

Ah yes, so we're on the thread exit path and we restore init xstate.
Sure, of course, that makes sense.

> And note! this is the yet another proof that init_fpu()->fpu_finit() is
> pointless. This (and almost all) users need fpu_alloc() only.

Right, applying.

Thanks for explaining!

:-)

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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