linux-kernel - Re: [PATCH 4/5] x86, NMI: Allow NMI reason io port (0x61) to be processed on any CPU

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Message-ID: <20101019183720.GN4140@redhat.com>
Date:	Tue, 19 Oct 2010 14:37:20 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Robert Richter <robert.richter@....com>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"ying.huang@...el.com" <ying.huang@...el.com>
Subject: Re: [PATCH 4/5] x86, NMI: Allow NMI reason io port (0x61) to be
 processed on any CPU

On Tue, Oct 19, 2010 at 06:25:07PM +0200, Robert Richter wrote:
> On 19.10.10 17:07:01, Robert Richter wrote:
> > On 15.10.10 22:22:17, Don Zickus wrote:
> > > From: Huang Ying <ying.huang@...el.com>
> > > 
> > > In original NMI handler, NMI reason io port (0x61) is only processed
> > > on BSP. This makes it impossible to hot-remove BSP. To solve the
> > > issue, a raw spinlock is used to make the port can be processed on any
> > > CPU.
> > > 
> > > Signed-off-by: Huang Ying <ying.huang@...el.com>
> > > Signed-off-by: Don Zickus <dzickus@...hat.com>
> > > ---
> > >  arch/x86/kernel/traps.c |   45 +++++++++++++++++++++++++--------------------
> > >  1 files changed, 25 insertions(+), 20 deletions(-)
> 
> > > @@ -400,28 +405,28 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> > >  		return;
> > >  
> > >  	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
> > > -	cpu = smp_processor_id();
> > > -	/* Only the BSP gets external NMIs from the system. */
> > > -	if (!cpu) {
> > > -		reason = get_nmi_reason();
> > > -		if (reason & NMI_REASON_MASK) {
> > > -			if (notify_die(DIE_NMI, "nmi", regs, reason, 2, SIGINT)
> > > -			    == NOTIFY_STOP)
> > > -				return;
> > > -			if (reason & NMI_REASON_SERR)
> > > -				pci_serr_error(reason, regs);
> > > -			else if (reason & NMI_REASON_IOCHK)
> > > -				io_check_error(reason, regs);
> > > +	raw_spin_lock(&nmi_reason_lock);
> > 
> > What about using raw_spin_trylock() instead? We don't have to wait
> > here since we are already processing it by another cpu.
> 
> This would avoid a global lock and also deadlocking in case of a
> potential #gp in the nmi handler.

I would feel more comfortable with it too.  I can't find a reason where
trylock would do harm.

Cheers,
Don
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