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From: Junio C Hamano <gitster@pobox.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Johannes Sixt <j6t@kdbg.org>, Andrew Lutomirski <luto@mit.edu>,
        Christian Couder <christian.couder@gmail.com>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        git@vger.kernel.org, Shuang He <shuang.he@intel.com>
Subject: Re: AAARGH bisection is hard (Re: [2.6.39 regression] X locks up
 hard right after logging in)
References: <BANLkTi=kb_m-CfrpnD8qQTVYLGaDdgy_tg@mail.gmail.com>
 <BANLkTikMeyRTOB9q4PEAYWnZRZfk3wg=kQ@mail.gmail.com>
 <BANLkTi=dL+KyQ3Bm58_Uj4LP9WSpbzAfJA@mail.gmail.com>
 <BANLkTi=NdVUUZ=_bACzyeMGS3JWs0EMbWA@mail.gmail.com>
 <BANLkTimE2GkkhcFZtNrYZASWp0LDhUx=GQ@mail.gmail.com>
 <BANLkTinyzBnksHk_rt8K2pmg90q5WyZX3w@mail.gmail.com>
 <BANLkTinVT=9+-HhwXcyLBwrnhX9F9Qz3ww@mail.gmail.com>
 <4DCD79A0.7000500@kdbg.org>
 <BANLkTi=smoaARKyzWxFjid-E7qehmyAX8w@mail.gmail.com>
Date: Fri, 13 May 2011 11:48:54 -0700
In-Reply-To: <BANLkTi=smoaARKyzWxFjid-E7qehmyAX8w@mail.gmail.com> (Linus
 Torvalds's message of "Fri, 13 May 2011 11:41:46 -0700")
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> When you say that v2.6.38 is good, that means that everything that can
> be reached from 2.6.38 is good.
>
> NOT AT ALL the same thing as "git bisect requires v2.6.38" would be.
>
> The "requires v2.6.38" would basically say that anything that doesn't
> contain v2.6.38 is "off-limits". It's fine to call them "good", but
> that's not the same thing as "git bisect good v2.6.38".
>
> Why?
>
> Think about it. It's the "reachable from v2.6.38" vs "cannot reach
> v2.6.38" difference. That's a HUGE difference.

Could you please clarify "off-limits"?

Do you mean "anything before v2.6.38 did not even have this feature, so
the result of testing a version in that range does not give us any
information"?  The feature didn't even exist, so a bug can never trigger,
and seeing "good" from such a version does not mean everything reachable
from it is good?  Upon seeing "bad" result from a version before v2.6.38,
what can we conclude?  The breakage cannot possibly come from the feature
that is being checked, so the procedure to check itself is busted?


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