netdev - Question about locks in qdisc

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Message-ID: <65634d660904271248p133717aam55751fc435f9fd3e@mail.gmail.com>
Date:	Mon, 27 Apr 2009 12:48:57 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	netdev@...r.kernel.org
Subject: Question about locks in qdisc_restart

In qdisc_restart the qdisc lock is released before taking the
netif_tx_lock, and only acquired again after unlocking the
netif_tx_lock.  There's a comment with the function that
"qdisc_lock(q) and netif_tx_lock are mutually exclusive, if one is
grabbed, another must be free."  Can anyone tell me the motivation for
this restriction?  We are seeing some performance improvements by
holding the lock through qdisc_restart, and I'm not sure why this
would be bad to do.

Thanks,
Tom
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