linux-kernel - Re: [tip:sched/core] cpu: Remove incorrect BUG

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Date:	Tue, 23 Nov 2010 16:08:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, mingo@...hat.com, tglx@...utronix.de, mingo@...e.hu
Subject: Re: [tip:sched/core] cpu: Remove incorrect BUG_ON

On 11/23, Peter Zijlstra wrote:
>
> Ah,. uhm,. you mean, not do anything at all?
>
> Dunno, really, let me try and read the code there.

Thanks. This is very minor of course, but it would be nice to
undestand the reason. To me it looks unneeded, but I don't trust
myself.  (snippets from my previous email below).

With or without this change, even if we know that rq->idle is running
we can't know if it (say) already started play_dead_common() or not.

We are going to call __cpu_die(), afaics it should do the necessary
synchronization in any case.

For example, native_cpu_die() waits for cpu_state == CPU_DEAD in a
loop. Of course it should work in practice (it also does msleep),
but in theory there is no guarantee.

So. Can't we just remove this wait-loop? We know that rq->idle
will be scheduled "soon", I don't understand why it is necessary
to ensure that context_switch() has already happened.

Oleg.

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