linux-kernel - Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

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Date:	Thu, 26 May 2011 12:32:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Marc Zyngier <Marc.Zyngier@....com>, Ingo Molnar <mingo@...e.hu>,
	Frank Rowand <frank.rowand@...sony.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()"
 locks up on ARM

On Thu, 2011-05-26 at 15:29 +0800, Yong Zhang wrote:
> > Figuring out why the existing condition failed
> 
> Seems  'current' will change before/after switch_to since it's derived from
> sp register.
> So that means if interrupt come before we switch sp, 'p == current' will
> catch it, but if interrupt comes after we switch sp, we will lose a wake up.

Well, loosing a wakeup isn't the problem here (although it would be a
problem), the immediate problem is that we're getting stuck
(life-locked) in that while (p->on_cpu) loop.

But yes, I think that explains it, if the interrupts hits
context_switch() after current was changed but before clearing
p->on_cpu, we would life-lock in interrupt context.

Now we could of course go add in_interrupt() checks there, but that
would make this already fragile path more interesting, so I think I'll
stick with the proposed patch -- again provided it actually works.

Marc, any word on that?
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