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Date: Sat, 17 Mar 2007 17:28:03 -0700
From: "Kip Macy" <kip.macy@gmail.com>
To: "Robert Watson" <rwatson@freebsd.org>
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References: <45FA98DD.3080205@cs.rutgers.edu>
	<20070316141836.J60288@fledge.watson.org>
Cc: freebsd-net@freebsd.org, Aniruddha Bohra <bohra@cs.rutgers.edu>
Subject: Re: ether_input question
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> The reason the drivers drop their locks is
> that the network stack frequently holds locks over calls to driver output
> routines.  As a result, driver locks tend to follow network stack locks in the
> lock order--at least, for drivers that have a single lock covering both send
> and receive paths (quite common).  As a result, the driver must drop the
> driver lock before calling into the stack to avoid a lock order reversal.

Just to further clarify the corollary to this statement - drivers that
separately lock their softc, tx, and rx queues don't need to drop the
lock across if_input as there is no possibility of lock order reversal
between the input and the output path.

            -Kip