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Subject: Re: Q: preemptible kernel and interrupts consistency.
From: Robert Love <rml@tech9.net>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: linux-kernel@vger.kernel.org
In-Reply-To: <3D2DEB91.57FA34E6@tv-sign.ru>
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On Thu, 2002-07-11 at 13:33, Oleg Nesterov wrote:

> Documentation/preempt-locking.txt states, that
> disabled interrupts prevents preemption.
> 
> Well, unless process does not touch TIF_NEED_RESCHED.

Yes, you are right, if need_resched is set under you, you will preempt
off the last unlock, even if interrupts are disabled.

However, the only places that set need_resched like that are the
scheduler and they do so also under lock so we are safe.

Also, in your example, being in an interrupt handler bumps the
preempt_count so even the scenario you give will not cause a
preemption.  If we did not bump the unlock, then your example would give
a lot of "scheduling in interrupt" BUGs so we would know it ;-)

All that said, there is a bug: the send_reschedule IPI can set
need_resched on another CPU.  If the other CPU happens to have
interrupts disabled, we can in fact preempt.  I have a patch for this I
will submit shortly.

	Robert Love

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