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

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Message-ID: <04EAB7311EE43145B2D3536183D1A8445493D025@GSjpTKYDCembx31.service.hitachi.net>
Date:	Mon, 24 Aug 2015 01:52:26 +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 08/17/2015 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().

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;
> >>>
> >>>