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Date: Fri, 6 Mar 2015 14:46:01 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bp@suse.de>,
        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 1/1] x86/fpu: math_state_restore() should not blindly
 disable irqs
Message-ID: <20150306134601.GA11718@gmail.com>
References: <54F74F59.5070107@intel.com>
 <20150305195127.GA12657@redhat.com>
 <20150305195149.GB12657@redhat.com>
 <20150305201101.GA21571@gmail.com>
 <20150305212532.GA16890@redhat.com>
 <20150306075833.GA623@gmail.com>
 <20150306132634.GA20693@redhat.com>
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* Oleg Nesterov <oleg@redhat.com> wrote:

> On 03/06, Ingo Molnar wrote:
> >
> > * Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > > [...] The patch above looks "obviously safe", but perhaps I am
> > > paranoid too much...
> >
> > IMHO your hack above isn't really acceptable, even for a backport.
> > So lets test the patch below (assuming it's the right thing to do)
> > and move forward?
> 
> OK, but please note that this patch is not beckportable. If you think
> that -stable doesn't need this fix, then I agree.
> 
> If the caller is do_device_not_available(), then we can not enable
> irqs before __thread_fpu_begin() + restore_fpu_checking().
> 
> 1. Preemption in between can destroy ->fpu.state initialized by
>    fpu_finit(), __switch_to() will save the live (wrong) FPU state
>    again.
> 
> 2. kernel_fpu_begin() from irq right after __thread_fpu_begin() is
>    not nice too. It will do __save_init_fpu() and this overwrites
>    ->fpu.state too.
> 
> Starting from v4.0 it does kernel_fpu_disable(), but the older kernels
> do not.
> 
> Ingo, this code is really horrible and fragile. We need to cleanup it
> step-by-step, imho.

How about the patch from David Vrabel? That seems to solve the 
irq-disable problem too, right?

Thanks,

	Ingo