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Date: Fri, 13 Mar 2015 16:51:21 +0100
From: Borislav Petkov <bp@suse.de>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Dave Hansen <dave.hansen@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pekka Riikonen <priikone@iki.fi>, Rik van Riel <riel@redhat.com>,
        Suresh Siddha <sbsiddha@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Quentin Casasnovas <quentin.casasnovas@oracle.com>
Subject: Re: [PATCH 3/4] x86/fpu: use restore_init_xstate() instead of
 math_state_restore() on kthread exec
Message-ID: <20150313155121.GH31998@pd.tnic>
References: <54F74F59.5070107@intel.com>
 <20150311173346.GB5032@redhat.com>
 <20150311173449.GE5032@redhat.com>
 <20150313104801.GC31998@pd.tnic>
 <20150313144514.GC21603@redhat.com>
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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.
--