Re: [RFC] sysctl locking

From: Don Lewis (truckman_at_FreeBSD.org)
Date: 10/11/04

  • Next message: Sam: "Re: sys/net/netisr.c" 
    Date: Mon, 11 Oct 2004 12:30:24 -0700 (PDT)
To: ssouhlal@FreeBSD.org

    On 10 Oct, Suleiman Souhlal wrote:
    > Hello,
    >
    > In the past few days, I have managed to remove Giant from sysctl. The
    > patch is at
    > http://people.freebsd.org/~ssouhlal/testing/sysctl-locking.diff
    > This is the first time I ever do something like this, so I wouldn't be
    > surprised if I am doing something wrong.
    > Any suggestions/comments?
    >
    > green@ also pointed out that I might need to use memory barriers, or
    > some other synchronization primitive, when setting oid_owner to NULL in
    > sysctl_oid_disown, but I am unsure on how to do that..
    >
    > Thanks in advance for your help.

    There seems to be a lot of locking/unlocking overhead in the oid lookup
    and oid tree manipulation code. Doing the traversals at each level of
    the tree without holding a lock for the entire time makes me nervous,
    though I can't point to any specific problem. It might be better to
    just hold a single lock across then entire lookup, insertion, or
    deletion operation.

    What happens if:
            thread A owns an oid

            thread B, which wants to delete the oid, goes to sleep to wait
            for the oid

            thread C wants the oid and goes to sleep

            thread A releases the oid and wakes up thread B

            thread B deletes the oid

            thread C does ???

    I don't think that you can arbitrarily remove
            mtx_assert(&Giant, MA_OWNED)
    from the handlers. Some of the handlers probably need Giant for correct
    operation. It's probably possible to change these into
    mtx_lock()/mtx_unlock() pairs.

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