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    SubjectRe: [PATCH 3/7] barrier: a scalable synchonisation barrier
    On 01/31, Paul E. McKenney wrote:
    >
    > QRCU as currently written (http://lkml.org/lkml/2006/11/29/330) doesn't
    > do what you want, as it acquires the lock unconditionally. I am proposing
    > that synchronize_qrcu() change to something like the following:
    >
    > void synchronize_qrcu(struct qrcu_struct *qp)
    > {
    > int idx;
    >
    > smp_mb();
    >
    > if (atomic_read(qp->ctr[0]) + atomic_read(qp->ctr[1]) <= 1) {
    > smp_rmb();
    > if (atomic_read(qp->ctr[0]) +
    > atomic_read(qp->ctr[1]) <= 1)
    > goto out;
    > }
    >
    > mutex_lock(&qp->mutex);
    > idx = qp->completed & 0x1;
    > atomic_inc(qp->ctr + (idx ^ 0x1));
    > /* Reduce the likelihood that qrcu_read_lock() will loop */
    > smp_mb__after_atomic_inc();

    I almost forgot. Currently this smp_mb__after_atomic_inc() is not strictly
    needed, and the comment is correct. However, it becomes mandatory with your
    optimization. Without this barrier, it is possible that both checks above
    mutex_lock() will see the result of atomic_dec(), but not the atomic_inc().

    So, may I ask you to also update this comment?

    /*
    * Reduce the likelihood that qrcu_read_lock() will loop
    * AND
    * make sure the second re-check above will see the result
    * of atomic_inc() if it sees the result of atomic_dec()
    */

    Something like this, I hope you will make it better.

    And another note: this all assumes that STORE-MB-LOAD works "correctly", yes?
    We have other code which relies on that, should not be a problem.

    (Alan Stern cc'ed).

    Oleg.

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