netdev - Re: Kernel WARNING: at net/core/dev.c:1330 __netif

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Message-Id: <20080726.173434.48036095.davem@davemloft.net>
Date:	Sat, 26 Jul 2008 17:34:34 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jarkao2@...il.com
Cc:	johannes@...solutions.net, netdev@...eo.de, peterz@...radead.org,
	Larry.Finger@...inger.net, kaber@...sh.net,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-wireless@...r.kernel.org, mingo@...hat.com
Subject: Re: Kernel WARNING: at net/core/dev.c:1330
 __netif_schedule+0x2c/0x98()

From: Jarek Poplawski <jarkao2@...il.com>
Date: Sat, 26 Jul 2008 15:18:38 +0200

> I guess some additional synchronization will be added yet to prevent
> parallel freeze and especially unfreeze.

Yes, that could be a problem.  Using test_and_set_bit() can
guard the freezing sequence itself, but it won't handle
letting two threads of control freeze and unfreeze safely
without a reference count.

We want this thing to be able to be used flexbly, which means
we can't just assume that this is a short code sequence and
the unfreeze will come quickly.  That pretty much rules
out using a new lock around the operation or anything
like that.

So I guess we could replace the state bit with a reference
count.  It doesn't even need to be atomic since it is set
and tested under dev_queue->_xmit_lock

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