linux-kernel - Re: [PATCH] make-cancel_rearming_delayed

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Date:	Tue, 8 May 2007 15:07:48 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Chinner <dgc@....com>,
	David Howells <dhowells@...hat.com>,
	Gautham Shenoy <ego@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] make-cancel_rearming_delayed_work-reliable-fix

On Tue, May 08, 2007 at 04:02:21PM +0400, Oleg Nesterov wrote:
> On 05/08, Jarek Poplawski wrote:
> >
> > On Mon, May 07, 2007 at 02:34:20PM +0400, Oleg Nesterov wrote:
> > > On 05/07, Jarek Poplawski wrote:
...
> I am strongly against adding many different variants of cancel_xxx().
> With this patch the API is simple
> 
> 	- cancel_delayed_work() stops the timer
> 
> 	- cancel_rearming_delayed_work() stops the timer and work,
> 	  doesn't need cancel_work_sync/flush_workqueue

But most often there is a simple rearming (but not neccessarily
always) work for which the first is not enough and the second
an overkill (especially with system/workqueues loaded).

> When work->func() re-arms itself, both queue_work() and queue_delayed_work()
> may change CPU if CPU_DOWN_xxx happens. Note that this is possible
> even if we use freezer for cpu-hotplug, because the timer migrates to
> another CPU anyway.

Thanks for explanation - I try to think about this.

> >                                I'm not sure what exactly place
> > did you mean - if spinlocking in wait_on_work - maybe it's
> > a sign this place isn't optimal too: it seems to me, this all
> > inserting/waiting for completion could be done under existing
> > locks e.g. in run_workqueue (maybe with some flag?).
> 
> Sorry, can't understand this. Inserting uses existing lock, namely
> cwq->lock.

I meant held locks - maybe e.g. after seeing some flag set (or some
other global/per_cpu/atomic/whatever variable) in a processed work
insert_wq_barrier could be done in already locked place. Or maybe
the whole idea of these completions should be rethinked, for
something lighter (i.e. lockless)?

> 
> Just in case, I think wait_on_cpu_work() can check ->current_work without
> cwq->lock, but I am not sure yet. We can remove unneeded "new |= " from
> set_wq_data(), we can do a couple of other optimizations. However, there
> are already _a lot_ of workqueue changes in -mm tree. We can do this later.
> Right now my only concern is correctness.

I agree 100% - it's -mm, the old way is working, so why hurry?

Jarek P.
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