netdev - Re: [PATCH net-next-2.6] bnx2: remove cancel_work_sync() from remove

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Message-ID: <20101221105103.GA32744@htj.dyndns.org>
Date:	Tue, 21 Dec 2010 11:51:04 +0100
From:	Tejun Heo <tj@...nel.org>
To:	David Miller <davem@...emloft.net>
Cc:	mchan@...adcom.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] bnx2: remove cancel_work_sync() from
 remove_one

Hello, David.

On Mon, Dec 20, 2010 at 01:11:46PM -0800, David Miller wrote:
> It would but we can't just make the change over to del_timer_sync()
> otherwise we'd deadlock on netif_tx_lock().
> 
> But I think things might be OK as-is.
> 
> The timer is deleted by dev_deactivate_many() which resets the qdisc
> to the no-op qdisc.  Then it deletes the timer.
> 
> Any running timer will complete or see the no-op qdisc attached and
> return immediately.
> 
> synchronize_rcu() is then executed which guarentees completion.
> 
> Since both the watchdog timer itself and the del_timer() call run
> with netif_tx_lock() held, this makes sure the timer, once deleted,
> will only see the no-op qdisc and return immediately if it is
> amidst running, else it has already returned when the timer delete
> completes.
> 
> So we might be OK here.

Yeah, I agree the synchronize_rcu() there would guarantee the actual
timer completion but as it currently stands it looks a bit too subtle.
Maybe it's a good idea to add a big fat comment explaining that the
the timer is guaranteed to stop after close() and how it's guaranteed
through synchronize_rcu() at the moment?  Also, it might be better to
use synchronize_sched() there as timer synchronization through
synchronize_rcu() is more of a happy accident.

Thanks.

-- 
tejun
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