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

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Message-ID: <51F1C745.60800@acm.org>
Date:	Thu, 25 Jul 2013 19:48:05 -0500
From:	Corey Minyard <tcminyard@...il.com>
To:	"Zheng, Lv" <lv.zheng@...el.com>
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

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.

>
>> 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.

>
> Thanks for commenting.

No problem, thanks for working on this.

-corey
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