linux-kernel - RE: [PATCH 7/7] ipmi/kcs: Don't run the KCS state machine when it is KCS

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Date:	Tue, 25 Aug 2015 03:53:22 +0000
From:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
To:	"'minyard@....org'" <minyard@....org>
CC:	"openipmi-developer@...ts.sourceforge.net" 
	<openipmi-developer@...ts.sourceforge.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 7/7] ipmi/kcs: Don't run the KCS state machine when it
 is KCS_IDLE

> From: Corey Minyard [mailto:tcminyard@...il.com] On Behalf Of Corey Minyard
> 
> On 08/23/2015 08:52 PM, 河合英宏 / KAWAI，HIDEHIRO wrote:
> >> From: Corey Minyard [mailto:tcminyard@...il.com] On Behalf Of Corey Minyard
> >>
> >> On 08/17/2015 09:54 PM, 河合英宏 / KAWAI，HIDEHIRO wrote:
> >>>> From: Corey Minyard [mailto:tcminyard@...il.com] On Behalf Of Corey Minyard
> >>>>
> >>>> This patch will break ATN handling on the interfaces.  So we can't do this.
> >>> I understand.  So how about doing like this:
> >>>
> >>> 	/* All states wait for ibf, so just do it here. */
> >>> -	if (!check_ibf(kcs, status, time))
> >>> +	if (kcs->state != KCS_IDLE && !check_ibf(kcs, status, time))
> >>> 		return SI_SM_CALL_WITH_DELAY;
> >>>
> >>> I think it is not necessary to wait IBF when the state is IDLE.
> >>> In this way, we can also handle the ATN case.
> >> I think it would be more reliable to go up a level and add a timeout.
> > It may be so, but we should address this issue separately (at least
> > I think above solution reasonably solves the issue).
> >
> > This issue happens after all queued messages are processed or dropped
> > by timeout.  There is no current message.  So what should we set
> > a timeout against?  We can add a timeout into my new flush_messages(),
> > but that is meaningful only in panic context.  That doesn't help
> > in normal context; we would perform a busy loop of smi_event_handler()
> > and schedule() in ipmi_thread().
> 
> I'm a little confused here.  Is the problem that the ATN bit is stuck
> high?  If so, it's going to be really hard to work around this without
> breaking ATN handling.

Sorry for my insufficient explanation.  I assume the case where
IBF bit is always 1.  I don't know what happens when
BMC hangs up, but I guess IBF stays in 1 because my server's
BMC behaves as such while rebooting.

Regards,

Hidehiro Kawai

> >> One should
> >> be there, anyway.  I thought they were all covered, but I may have missed
> >> something.
> >>
> >> -corey
> >>
> >>> Regards,
> >>>
> >>> Hidehiro Kawai
> >>> Hitachi, Ltd. Research & Development Group
> >>>
> >>>> It's going to be extremely hard to recover if the BMC is not working
> >>>> correctly when a panic happens.  I'm not sure what can be done, but if
> >>>> you can fix it another way it would be good.
> >>>>
> >>>> -corey
> >>>>
> >>>> On 07/27/2015 12:55 AM, Hidehiro Kawai wrote:
> >>>>> If a BMC is unresponsive for some reason, it ends up completing
> >>>>> the requested message as an error, then kcs_event() is called once
> >>>>> to advance the state machine.  However, since the BMC is
> >>>>> unresponsive now, the status of the KCS interface may not be
> >>>>> idle.  As the result, the state machine can continue to run and
> >>>>> comsume CPU time indefinitely even if there is no more request
> >>>>> message.  Moreover, if this happens in run-to-completion mode
> >>>>> (i.e. context of panic_event()), the kernel hangs up.
> >>>>>
> >>>>> To fix this problem, this patch ignores kcs_event() call if there
> >>>>> is no request message to be processed.
> >>>>>
> >>>>> Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>
> >>>>> ---
> >>>>>  drivers/char/ipmi/ipmi_kcs_sm.c |    4 ++++
> >>>>>  1 file changed, 4 insertions(+)
> >>>>>
> >>>>> diff --git a/drivers/char/ipmi/ipmi_kcs_sm.c b/drivers/char/ipmi/ipmi_kcs_sm.c
> >>>>> index 8c25f59..0e187fb 100644
> >>>>> --- a/drivers/char/ipmi/ipmi_kcs_sm.c
> >>>>> +++ b/drivers/char/ipmi/ipmi_kcs_sm.c
> >>>>> @@ -353,6 +353,10 @@ static enum si_sm_result kcs_event(struct si_sm_data *kcs, long time)
> >>>>>  	if (kcs_debug & KCS_DEBUG_STATES)
> >>>>>  		printk(KERN_DEBUG "KCS: State = %d, %x\n", kcs->state, status);
> >>>>>
> >>>>> +	/* We don't want to run the state machine when the state is IDLE */
> >>>>> +	if (kcs->state == KCS_IDLE)
> >>>>> +		return SI_SM_IDLE;
> >>>>> +
> >>>>>  	/* All states wait for ibf, so just do it here. */
> >>>>>  	if (!check_ibf(kcs, status, time))
> >>>>>  		return SI_SM_CALL_WITH_DELAY;
> >>>>>
> >>>>>