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    SubjectRe: [rfc] "fair" rw spinlocks
    On Sat, Nov 28, 2009 at 12:15:07PM +0100, Andi Kleen wrote:
    > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
    > >
    > > The usual trick would be to keep per-fair-rwlock state in per-CPU
    > > variables. If it is forbidden to read-acquire one nestable fair rwlock
    > > while read-holding another, then this per-CPU state can be a single
    > > pointer and a nesting count. On the other hand, if it is permitted to
    > > read-acquire one nestable fair rwlock while holding another, then one
    > > can use a small per-CPU array of pointer/count pairs.
    >
    > The problem is that in preemptible kernels kernel threads can switch
    > CPUs all the time. How would you sync the per CPU state then?

    In preemptible kernels, put the state into the task structure. Perhaps do
    this in non-preemptible kernels too, just to save a bit of source code.

    Thanx, Paul


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