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

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Message-Id: <20101220.131146.115941299.davem@davemloft.net>
Date:	Mon, 20 Dec 2010 13:11:46 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	tj@...nel.org
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

From: Tejun Heo <tj@...nel.org>
Date: Wed, 15 Dec 2010 14:52:29 +0100

> After looking through the code, I don't think this is necessarily
> correct.  ->ndo_close() doesn't guarantee that the watchdog timer has
> finished running (the timer is deleted with del_timer() not
> del_timer_sync()).  ie. the watchdog timer could still be running
> after ->ndo_close() and may schedule reset_task.  If remove_one
> doesn't flush the task, it may still be running when remove_one() is
> called.
> 
> David, am I missing something?  Wouldn't it cleaner to guarantee that
> ->ndo_close() is called with the guarantee that the watchdog timer is
> not running anymore?

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.
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