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Date: Thu, 5 Mar 2015 22:25:32 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
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: <20150305212532.GA16890@redhat.com>
References: <54F74F59.5070107@intel.com> <20150305195127.GA12657@redhat.com> <20150305195149.GB12657@redhat.com> <20150305201101.GA21571@gmail.com>
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On 03/05, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -774,7 +774,10 @@ void math_state_restore(void)
> >  	struct task_struct *tsk = current;
> >
> >  	if (!tsk_used_math(tsk)) {
> > -		local_irq_enable();
> > +		bool disabled = irqs_disabled();
> > +
> > +		if (disabled)
> > +			local_irq_enable();
> >  		/*
> >  		 * does a slab alloc which can sleep
> >  		 */
> > @@ -785,7 +788,9 @@ void math_state_restore(void)
> >  			do_group_exit(SIGKILL);
> >  			return;
> >  		}
> > -		local_irq_disable();
> > +
> > +		if (disabled)
> > +			local_irq_disable();
> >  	}
>
> Yuck!
>
> Is there a fundamental reason why we cannot simply enable irqs and
> leave them enabled? Math state restore is not atomic and cannot really
> be atomic.

You know, I didn't even try to verify ;) but see below.

Most probably we can simply enable irqs, yes. But what about older kernels,
how can we check?

And let me repeat, I strongly believe that this !tsk_used_math() case in
math_state_restore() must die. And unlazy_fpu() in init_fpu(). And both
__restore_xstate_sig() and flush_thread() should not use math_state_restore()
at all. At least in its current form.

But this is obviously not -stable material.

That said, I'll try to look into git history tomorrow. The patch above
looks "obviously safe", but perhaps I am paranoid too much...

Oleg.