linux-kernel - Re: [kernel.org users] [KORG] Panics on master backend

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Message-ID: <20110829143855.GA29953@redhat.com>
Date:	Mon, 29 Aug 2011 16:38:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frank Rowand <frank.rowand@...sony.com>,
	linux-kernel <linux-kernel@...r.kernel.org>, users@...nel.org,
	hch <hch@...radead.org>, "yong.zhang0" <yong.zhang0@...il.com>,
	scameron@...rdog.cce.hp.com, Jens Axboe <jaxboe@...ionio.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>
Subject: Re: [kernel.org users] [KORG] Panics on master backend

On 08/29, Peter Zijlstra wrote:
>
> On Thu, 2011-08-25 at 15:54 +0200, Oleg Nesterov wrote:
> > > Isn't us holding ->pi_lock sufficient to stabilize task_cpu()? If its a
> > > running task the initial ->state check would have failed,
> >
> > Of course it is not TASK_RUNNING, but it can be running or not.
> >
> > > and thus its a
> > > proper wakeup when we get here and thus ->pi_lock is serializing things.
> >
> > I am not sure. If ->on_rq is true, we need rq->lock. Say, pull_task() can
> > change its cpu.
>
> If its !TASK_RUNNING but ->on_rq is true, it must be current, and
> pull_task() will never move current around.

You meant task_running() ? But it can be preempted in !TASK_RUNNING
state.

Oleg.

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