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Re: [PATCH v3] powerpc: strstr optimization
- From: "Carlos O'Donell" <carlos at redhat dot com>
- To: Joseph Myers <joseph at codesourcery dot com>, Tulio Magno Quites Machado Filho <tuliom at linux dot vnet dot ibm dot com>, Steve Munroe <sjmunroe at us dot ibm dot com>, Florian Weimer <fweimer at redhat dot com>
- Cc: OndÅej BÃlka <neleai at seznam dot cz>, GNU C Library <libc-alpha at sourceware dot org>, Rajalakshmi Srinivasaraghavan <raji at linux dot vnet dot ibm dot com>
- Date: Wed, 22 Jul 2015 13:55:40 -0400
- Subject: Re: [PATCH v3] powerpc: strstr optimization
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- References: <558A5642 dot 5020107 at linux dot vnet dot ibm dot com> <558A5761 dot 2000409 at linux dot vnet dot ibm dot com> <87oajpm8nc dot fsf at totoro dot br dot ibm dot com> <871tgijuri dot fsf at linux dot vnet dot ibm dot com> <55A6FE3F dot 6090701 at redhat dot com> <55A70B70 dot 6090607 at redhat dot com> <20150716195538 dot GA5140 at domone> <55A8110C dot 7000209 at redhat dot com> <alpine dot DEB dot 2 dot 10 dot 1507221607370 dot 21570 at digraph dot polyomino dot org dot uk>
On 07/22/2015 12:12 PM, Joseph Myers wrote:
> On Thu, 16 Jul 2015, Carlos O'Donell wrote:
>
>> On 07/16/2015 03:55 PM, OndÅej BÃlka wrote:
>>> On Wed, Jul 15, 2015 at 09:40:00PM -0400, Carlos O'Donell wrote:
>>>> On 07/15/2015 08:43 PM, Carlos O'Donell wrote:
>>>>>> May I proceed with this commit?
>>>>>
>>>>> Yes, please commit this for 2.22.
>>>>
>>>> For the record I trust IBM to make sure these patches make incremental
>>>> improvements in performance even if they are not the best possible
>>>> performance as pointed out by Ondrej Bilka.
>>>>
>>> Sorry Carlos, your trust is misplaced. This patch wasn't reviewed at
>>> all. I did that as test how much we could test IBM to verify patches.
>>>
>>> I pointed out that it could have possibly quadratic behaviour which
>>> still does. So please don't accept unreviewed patches next time.
>>
>> They showed cases for which the code does go faster and objectively
>> so using the microbenchmark, and that's a win for now. Please continue
>> to work with IBM to remove the quadratic worst case.
>>
>> Tulio, You will need to work out why you have quadratic worst case.
>> It's certainly something we try to avoid. Did you make a particular
>> decision not to avoid it?
>
> If there's a quadratic worst case newly introduced for 2.22, I'd consider
> that a security hole (denial of service) that needs to block the release
> of 2.22 until it's fixed (possibly by removing the implementation in
> question).
Joseph,
We have had quadratic worse case in our string routines for years without
it blocking a release. I agree that it is not the best case for a release
to have such behaviour, but should it block this release?
Florian,
I would like to hear your opinion on whether you might consider this a
security issue, or simply a QoI issue.
Tulio,
Could you please review the possibility of quadratic behaviour and respond
prompty? I don't want this to hold up the release.
Cheers,
Carlos.