duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c

From: Luigi Rizzo (rizzo_at_icir.org)
Date: 08/17/05

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    Date: Tue, 16 Aug 2005 17:05:19 -0700
To: arch@freebsd.org, net@freebsd.org

    [apologies for the cross post but it belongs both to arch and net.]

    I notice that net/pfil.c and netinet/ip_fw2.c have two copies of
    aisimilar but slightly different implementation of
    multiple-reader/single-writer locks, which brings up the question(s):

    1. should we rather put this code in the generic kernel code so that other
       subsystems could make use of it ? E.g. the routing table is certainly
       a candidate,

    and especially

    2. should we implement it right ?

       Both implementations are subject to starvation for the writers
       (which is indeed a problem here, because we might want to modify
       a ruleset and be prevented from doing it because of incoming traffic
       that keeps readers active).
       Also the PFIL_TRY_WLOCK will in fact be blocking if a writer
       is already in - i have no idea how problematic is this in the
       way it is actually used.

            cheers
            luigi
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