linux-kernel - Re: [RFC PATCH(Experimental) 2/4] Revert changes to workqueue.c

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Date:	Wed, 21 Feb 2007 20:07:51 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Srivatsa Vaddagiri <vatsa@...ibm.com>, akpm@...l.org,
	paulmck@...ibm.com, mingo@...e.hu, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	rjw@...k.pl
Subject: Re: [RFC PATCH(Experimental) 2/4] Revert changes to workqueue.c

On Wed, Feb 21, 2007 at 05:30:10PM +0300, Oleg Nesterov wrote:
> On 02/21, Srivatsa Vaddagiri wrote:
> >
> > On Tue, Feb 20, 2007 at 11:09:36PM +0300, Oleg Nesterov wrote:
> > > > Which caller are you referring to here? Maybe we can decide on the
> > > > option after we see the users of flush_workqueue() in DOWN_PREPARE.
> > > 
> > > mm/slab.c:cpuup_callback()
> > 
> > The cancel_rearming_delayed_work, if used as it is in cpuup_callback,
> > will require that we send DOWN_PREPARE before freeze_processes().
> > 
> > But ..I am wondering if we can avoid doing cancel_rearming_delayed_work
> > (and thus flush_workqueue) in CPU_DOWN_PREPARE of slab.c. Basically,
> > 
> > mm/slab.c:
> > 
> > 	CPU_DOWN_PREPARE:	/* All processes frozen now */
> > 		cancel_delayed_work(&per_cpu(reap_work, cpu).timer);
> > 		del_work(&per_cpu(reap_work, cpu).work);
> > 		break;
> > 
> > 
> > At the point of CPU_DOWN_PREPARE, keventd should be frozen and hence
> > del_work() is a matter of just deleting the work from cwq->worklist.
> 
> Agreed. Note that we don't need the new "del_work". It is always safe to
> use cancel_work_sync() if we know that the workqueue is frozen, it won't
> block. We can also do
> 
> 	if (!cancel_delayed_work())
> 		cancel_work_sync();
> 
> but it is ok to do cancel_work_sync() unconditionally.

True. But this might be a one off solution for slab. However, if someone
in the future might require to do a flush_workqueue from
CPU_DOWN_PREPARE context, we would need to find a new workaround.

So, I'll try running CPU_DOWN_PREPARE and CPU_UP_PREPARE from
a non frozen context to check if there are any potential problems.
Hopfully there shouldn't be (m)any!
> 
> Oleg.
> 
thanks and regards
gautham.

-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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