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

From: Julian Elischer (julian_at_elischer.org)
Date: 08/17/05

  • Next message: gnn_at_freebsd.org: "Re: Special schedulers, one CPU only kernel, one only userland" 
    Date: Tue, 16 Aug 2005 20:20:17 -0700
To: Max Laier <max@love2party.net>

    Max Laier wrote:
    > On Wednesday 17 August 2005 02:05, Luigi Rizzo wrote:
    >
    >>[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,
    >
    >
    > I have asked this several time on -arch and IRC, but never found anyone
    > willing to pursue it. However, the problem is ...
    >
    >
    >>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.
    >
    >
    > ... really this. I didn't find a clean way out of the starvation issue. What
    > I do for pfil is that I set a flag and simply stop serving[2] shared requests
    > once a writer waits for the lock. If a writer can't sleep[1] then we return
    > EBUSY and don't. However, for pfil it's almost ever safe to assume that a
    > write may sleep (as it is for most instances of this kind of sx-lock where
    > you have BIGNUMxreads:1xwrite).
    >
    > [1] Note that there is a *big* difference between blocking and sleeping.
    > These two are usually confused. While it is almost always okay to block it
    > is seldom okay to sleep. The existing sx(9) api has the problem that it
    > *sleeps* in the shared path which renders it unusable for this usecase (as we
    > might be holding other locks and must not sleep in the shared path).
    > However, sleeping in the shared path is one (?the only?) way out of the
    > starvation problem - other than a problem specific as done for pfil.
    >
    > [2] See pfil(9) BUGS.

    netgraph has yet another implementation of R/W locks.
    It relies on the fact that every lock action is done on behalf of
    a command request or a data processing request, each of which is
    queueable, and each RW lock is associated with a queue.

    Instead of blocking, the item is queued instead for later processing.

    >

    _______________________________________________
    freebsd-arch@freebsd.org mailing list
    http://lists.freebsd.org/mailman/listinfo/freebsd-arch
    To unsubscribe, send any mail to "freebsd-arch-unsubscribe@freebsd.org"

  • Next message: gnn_at_freebsd.org: "Re: Special schedulers, one CPU only kernel, one only userland"

    Relevant Pages