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From: Robert Watson <rwatson@FreeBSD.org>
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To: Julian Elischer <julian@elischer.org>
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Cc: arch@freebsd.org, Poul-Henning Kamp <phk@phk.freebsd.dk>
Subject: Re: need for another mutex type/flag?
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On Sun, 25 Jan 2009, Julian Elischer wrote:

> Even purely documentary would be good but given the option, I'd like it to 
> scream when Witness is enabled and you try get another mutex....
>
> there are certain contexts (e.g. in most netgraph nodes) where we really 
> want the authors to only take such locks and taking more unpredictable 
> mutexes plays havoc with netgraph response times as a system as a whole, 
> since if one node blocks, teh thread doesn't get to go off and service other 
> nodes.

I thought lots of Netgraph nodes liked to do things like call m_pullup() and 
m_prepend()?  Disallowing acquiring mutexes will prevent them from doing that.

Robert N M Watson
Computer Laboratory
University of Cambridge