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Subject: Re: [PATCH] BUG(): sched.c: Line 944
From: Robert Love <rml@tech9.net>
To: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@transmeta.com>, linux-kernel@vger.kernel.org
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Date: 17 Sep 2002 14:27:52 -0400
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On Tue, 2002-09-17 at 05:57, Ingo Molnar wrote:

> We *must* use the schedule() check to debug preemption bugs, or we wont
> have usable preemption in 2.6, i dont really understand why your are not
> happy that we have such a great tool. In fact we should also add other
> debugging bits, like 'check for !0 preemption count in smp_processor_id()'
> , and the underflow checks that caught the IDE bug. These are all bits
> that help the elimination of preemption bugs which are also often SMP
> bugs, on plain UP boxes.

Hm, sorry if I sound like I do not want something "so great".  I do.  I
just do not ever want to compromise the existing code... I would much
prefer to say "wow we cannot do this cleanly now, let's wait until we
figure out a clean way".

Anyhow, one of us is confused.  How can this in_atomic() test _ever_
catch a preemption bug?  We cannot enter the scheduler off kernel
preemption unless preempt_count==0.  This is a test to catch bugs in
other parts of the kernel, e.g. where code explicitly calls schedule()
while holding a lock.

	Robert Love

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