linux-kernel - Re: [PATCH 2/3] ACPI: fix acpi_parse_entries

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Message-ID: <00e1e2ec-b7e1-606d-3faa-271560864d0e@redhat.com>
Date:	Fri, 1 Jul 2016 15:55:43 -0600
From:	Al Stone <ahs3@...hat.com>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 2/3] ACPI: fix acpi_parse_entries_array() so it traverses
 all subtables

On 07/01/2016 03:46 PM, Rafael J. Wysocki wrote:
> On Fri, Jul 1, 2016 at 11:41 PM, Al Stone <ahs3@...hat.com> wrote:
>> On 07/01/2016 03:32 PM, Rafael J. Wysocki wrote:
>>> On Fri, Jul 1, 2016 at 11:21 PM, Al Stone <ahs3@...hat.com> wrote:
>>>> Without this patch, the acpi_parse_entries_array() function will return
>>>> the very first time there is any error found in either the array of
>>>> callback functions or if one of the callbacks returns an non-zero value.
>>>> However, the array of callbacks could still have valid entries further
>>>> on in the array, or the callbacks may be able to process subsequent
>>>> subtables without error.  The change here makes the function consistent
>>>> with its description so that it will properly return the sum of all
>>>> matching entries for all proc handlers, instead of stopping abruptly
>>>> as it does today.
>>>
>>> I'm not sure I follow.
>>>
>>> You seem to be saying that the function should process all of the
>>> subtables etc even though errors have been found for some of them, but
>>> it still will return an error in the end if there are any errors.  How
>>> exactly does it help to continue processing in case of an error, then?
>>
>> The use case I have in mind is to simply count all of the subtables of
>> a certain type.  If for some reason, the callback -- or any other callback
>> -- fails, the traversal of all the subtables stops immediately.  So, I
>> could have two callbacks, and if the first one fails on the first subtable
>> of its type, traversal stops.  The count for the second callback will be
>> zero which may or may not be correct.
> 
> It will be zero, because the callback has not been invoked at all.
> Why is this incorrect?
> 

Because there could be additional subtables after the one causing a failure
that the second callback could have counted; e.g., if the failure is on the
first subtable of 20 in the MADT, the following 19 would be ignored, even if
they were all the right subtype for the second callback.

-- 
ciao,
al
-----------------------------------
Al Stone
Software Engineer
Red Hat, Inc.
ahs3@...hat.com
-----------------------------------