linux-kernel - Re: workqueue deadlock

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Message-Id: <20061210220343.e58614ae.akpm@osdl.org>
Date:	Sun, 10 Dec 2006 22:03:43 -0800
From:	Andrew Morton <akpm@...l.org>
To:	vatsa@...ibm.com
Cc:	mingo@...e.hu, bjorn.helgaas@...com, linux-kernel@...r.kernel.org,
	myron.stowe@...com, axboe@...nel.dk, dipankar@...ibm.com,
	ego@...ibm.com
Subject: Re: workqueue deadlock

> On Mon, 11 Dec 2006 11:15:45 +0530 Srivatsa Vaddagiri <vatsa@...ibm.com> wrote:
> On Sun, Dec 10, 2006 at 04:16:00AM -0800, Andrew Morton wrote:
> > One quite different way of addressing all of this is to stop using
> > stop_machine_run() for hotplug synchronisation and switch to the swsusp
> > freezer infrastructure: all kernel threads and user processes need to stop
> > and park themselves in a known state before we allow the CPU to be removed.
> > lock_cpu_hotplug() becomes a no-op.
> 
> Well ...you still need to provide some mechanism for stable access to
> cpu_online_map in blocking functions (ex: do_event_scan_all_cpus). 
> Freezing-tasks/Resuming-them-after-hotp-unplug is definitely not one of them 
> (when they resume, online_map would have changed under their feet).

Problems will only occur if a process is holding some sort of per-cpu state
across a call to try_to_freeze().  Surely nobody does that.
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