Re: Race condition in mb_free_ext()?

From: John Baldwin (jhb_at_FreeBSD.org)
Date: 03/02/05

  • Next message: Igor Pokrovsky: "Re: Dynamically Linked Library Problem (maybe)" 
    To: Bosko Milekic <bmilekic@technokratis.com>
Date: Wed, 2 Mar 2005 10:16:36 -0500

    On Tuesday 01 March 2005 06:49 pm, Bosko Milekic wrote:
    > You know, the more and more we run into these problems specific to how
    > reference counting is performed, I keep wondering whether some cleverly
    > defined macros for dealing with common reference counting operations
    > would be worthwhile. I'm not saying "introduce a refcount_t type and a
    > full-blown abstraction," but some template-like stuff might be useful.
    > It took a while just to get the refcount decrement on free path free of
    > races on SMP, and that's only in the mbuf code.

    Yeah, I have those simple refcount_foo() macros that operate on ints that
    would work for here and things like ucreds. Being macros, each arch can
    override if they have a better native instruction (xadd on x86, fetchadd on
    ia64, etc.)

    > -Bosko
    >
    > On Tue, Mar 01, 2005 at 03:14:38PM -0800, Kris Kennaway wrote:
    > > On Tue, Mar 01, 2005 at 06:04:27PM -0500, Bosko Milekic wrote:
    > > > This does not appear to explain the livelock.
    > >
    > > alc and dwhite tracked it down to a missing volatile causing gcc to
    > > mis-optimize the loop:
    > >
    > > - cnt = *(m->m_ext.ref_cnt);
    > > + cnt = *(volatile u_int *)(m->m_ext.ref_cnt);
    > >
    > > I'm currently testing that.
    > >
    > > Kris 

    -- 
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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