netdev - Re: [PATCH] pkt_sched: Fix lockdep warning on est_tree_lock in gen

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Message-Id: <20100902.135929.15241881.davem@davemloft.net>
Date:	Thu, 02 Sep 2010 13:59:29 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	jarkao2@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH] pkt_sched: Fix lockdep warning on est_tree_lock in
 gen_estimator

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 02 Sep 2010 22:53:33 +0200

> Maybe after a night, I'll understand ?
> 
> est_tree_lock is only taken by process context, and I dont see what can
> be the problem. Are you sure its not a lockdep false positive, or that
> the real bug is elsewhere ?
> 
> Sure, we can block BH everywhere, it will reduce bugs and lockdep
> alarms, but I would like to understand before, why its needed.
> 
> If you believe est_tree_lock can be taken by a softirq handler, please
> tell me ;)

Hint: est->stats_lock is actually a pointer to a qdisc->lock

Therefore we must comply with the rules of qdisc locking if we wish
to take est->stats_lock in conjunction with est_tree_lock.

The strange mention of qdisc_tx_lock changing state inside of
est_timer() in the lockdep warning should have been the clue :-)
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