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    SubjectRe: [rfc] "fair" rw spinlocks
    On Sat, Nov 28, 2009 at 09:30:18AM -0800, Linus Torvalds wrote:
    >
    >
    > On Mon, 23 Nov 2009, Nick Piggin wrote:
    > >
    > > Last time this issue came up that I could see, I don't think
    > > there were objections to making rwlocks fair, the main
    > > difficulty seemed to be that we allow reentrant read locks
    > > (so a write lock waiting must not block arbitrary read lockers).
    >
    > We have at least one major rwlock user - tasklist_lock or whatever. And
    > that one definitely depends on being able to do 'rwlock()' in an
    > interrupt, without other rwlock'ers having to disable irq's (even if there
    > might be a new writer coming in on another cpu).

    In other words, any fair rwlock must unconditionally grant read access
    to any CPU that already read-holds the lock, regardless of the state of
    any writer wannabees. Or is there another requirement I am missing?

    > That usage case _might_ be turned into RCU or something similar, in which
    > case I don't think any major rwlock users remain. However, if that's the
    > case, then why should anybody care about fairness any more either?

    Indeed, RCU goes a step further, permitting new readers unconditionally,
    regardless of writer state. ;-)

    > So as far as I can tell, we have only one real user of rwlocks where
    > livelocks might be relevant, but that one real user absolutely _requires_
    > the unfair behavior.

    But the required unfairness is limited to unconditionally granting
    recursive read requests, right? If I understand correctly, if a given
    CPU does not already read-hold the lock, then we can safely make that
    CPU wait for a writer that might otherwise be starved. Again, is there
    another requirement that I am missing?

    Thanx, Paul


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