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

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Message-ID: <20070221143010.GC134@tv-sign.ru>
Date:	Wed, 21 Feb 2007 17:30:10 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
Cc:	Gautham R Shenoy <ego@...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 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.

Oleg.

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