linux-kernel - RE: [PATCH 03/13] ACPI/IPMI: Fix race caused by the unprotected ACPI IPMI transfers

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Date:	Fri, 26 Jul 2013 01:30:05 +0000
From:	"Zheng, Lv" <lv.zheng@...el.com>
To:	"minyard@....org" <minyard@....org>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
	"Brown, Len" <len.brown@...el.com>,
	"Zhao, Yakui" <yakui.zhao@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"openipmi-developer@...ts.sourceforge.net" 
	<openipmi-developer@...ts.sourceforge.net>
Subject: RE: [PATCH 03/13] ACPI/IPMI: Fix race caused by the unprotected
 ACPI IPMI transfers

> From: Corey Minyard [mailto:tcminyard@...il.com]
> Sent: Friday, July 26, 2013 8:48 AM
> 
> On 07/25/2013 07:16 PM, Zheng, Lv wrote:
> >>
> >> If I understand this correctly, the problem would be if:
> >>
> >> rem_time = wait_for_completion_timeout(&tx_msg->tx_complete,
> >>                                           IPMI_TIMEOUT);
> >>
> >> returns on a timeout, then checks msg_done and races with something
> >> setting msg_done.  If that is the case, you would need the smp_rmb()
> >> before checking msg_done.
> >>
> >> However, the timeout above is unnecessary.  You are using
> >> ipmi_request_settime(), so you can set the timeout when the IPMI
> >> command fails and returns a failure message.  The driver guarantees a
> >> return message for each request.  Just remove the timeout from the
> >> completion, set the timeout and retries in the ipmi request, and the
> >> completion should handle the barrier issues.
> > It's just difficult for me to determine retry count and timeout value, maybe
> retry=0, timeout=IPMI_TIMEOUT is OK.
> > The code of the timeout completion is already there, I think the quick fix code
> should not introduce this logic.
> > I'll add a new patch to apply your comment.
> 
> Since it is a local BMC, I doubt a retry is required.  That is probably fine.  Or
> you could set retry=1 and timeout=IPMI_TIMEOUT/2 if you wanted to be more
> sure, but I doubt it would make a difference.  The only time you really need to
> worry about retries is if you are resetting the BMC or it is being overloaded.

I think for ACPI IPMI operation region, retries can be implemented in the ASL codes by the BIOS.
I'll check if retry=0 is correct.

> 
> >
> >> Plus, from a quick glance at the code, it doesn't look like it will
> >> properly handle a situation where the timeout occurs and is handled
> >> then the response comes in later.
> > PATCH 07 fixed this issue.
> > Here we just need the smp_rmb() or holding tx_msg_lock() around the
> acpi_format_ipmi_response().
> 
> If you apply the fix like I suggest, then the race goes away.  If there's no
> timeout and it just waits for the completion, things get a lot simpler.

Exactly.  I'll try to apply this in this patch, then the PATCH 07 is also need to be re-worked.

Thanks and best regards
-Lv


> >
> > Thanks for commenting.
> 
> No problem, thanks for working on this.
> 
> -corey