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Re: [PATCH v3] powerpc: strstr optimization
- From: "Tulio Magno Quites Machado Filho" <tuliom at linux dot vnet dot ibm dot com>
- To: "Carlos O'Donell" <carlos at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>
- Cc: Steve Munroe <sjmunroe at us dot ibm dot com>, Florian Weimer <fweimer at redhat dot com>, 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 15:29:12 -0300
- Subject: Re: [PATCH v3] powerpc: strstr optimization
- Authentication-results: sourceware.org; auth=none
- 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> <55AFD91C dot 30404 at redhat dot com>
"Carlos O'Donell" <firstname.lastname@example.org> writes:
> 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
Could you elaborate why a quadratic worst case in a string function can be
considered a denial of service, please?
> Could you please review the possibility of quadratic behaviour and respond
> prompty? I don't want this to hold up the release.
I confirm this algorithm does have a quadratic worst case, which appears if
needle <= 2048.
If the needle > 2048, it falls back to the previous implementation.