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    SubjectRe: [openib-general] Re: [PATCH 10/13] [RFC] ipath verbs, part 1
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    The quick answer is the qp_list is traversed w/o the lock held in
    ipath_ib_rcv(). The intent is to be able to do a lookup on the GID
    to get a reference to the struct ipath_mcast and then walk the qp_list
    w/o locks being held while processing the received packets at
    interrupt level.


    On Sun, 2005-12-18 at 11:59 -0800, Paul E. McKenney wrote:
    > On Fri, Dec 16, 2005 at 03:48:55PM -0800, Roland Dreier wrote:
    > > First half of ipath verbs driver
    >
    > Some RCU-related questions interspersed. Basic question is "where is
    > the lock-free read-side traversal?"
    >
    > Thanx, Paul
    >
    > > ---
    > >
    > > drivers/infiniband/hw/ipath/ipath_verbs.c | 3244 +++++++++++++++++++++++++++++
    > > 1 files changed, 3244 insertions(+), 0 deletions(-)
    > > create mode 100644 drivers/infiniband/hw/ipath/ipath_verbs.c
    ...
    > > +/*
    > > + * Insert the multicast GID into the table and
    > > + * attach the QP structure.
    > > + * Return zero if both were added.
    > > + * Return EEXIST if the GID was already in the table but the QP was added.
    > > + * Return ESRCH if the QP was already attached and neither structure was added.
    > > + */
    > > +static int ipath_mcast_add(struct ipath_mcast *mcast,
    > > + struct ipath_mcast_qp *mqp)
    > > +{
    > > + struct rb_node **n = &mcast_tree.rb_node;
    > > + struct rb_node *pn = NULL;
    > > + unsigned long flags;
    > > +
    > > + spin_lock_irqsave(&mcast_lock, flags);
    > > +
    > > + while (*n) {
    > > + struct ipath_mcast *tmcast;
    > > + struct ipath_mcast_qp *p;
    > > + int ret;
    > > +
    > > + pn = *n;
    > > + tmcast = rb_entry(pn, struct ipath_mcast, rb_node);
    > > +
    > > + ret = memcmp(mcast->mgid.raw, tmcast->mgid.raw,
    > > + sizeof(union ib_gid));
    > > + if (ret < 0) {
    > > + n = &pn->rb_left;
    > > + continue;
    > > + }
    > > + if (ret > 0) {
    > > + n = &pn->rb_right;
    > > + continue;
    > > + }
    > > +
    > > + /* Search the QP list to see if this is already there. */
    > > + list_for_each_entry_rcu(p, &tmcast->qp_list, list) {
    >
    > Given that we hold the global mcast_lock, how is RCU helping here?

    Its not really. I'm just trying to be consistent where ever the
    qp_list is traversed.

    > Is there a lock-free read-side traversal path somewhere that I am
    > missing?

    The lock free traversal is in ipath_ib_rcv() which is an interrupt
    routine.

    > > + if (p->qp == mqp->qp) {
    > > + spin_unlock_irqrestore(&mcast_lock, flags);
    > > + return ESRCH;
    > > + }
    > > + }
    > > + list_add_tail_rcu(&mqp->list, &tmcast->qp_list);
    >
    > Ditto...
    >
    > > + spin_unlock_irqrestore(&mcast_lock, flags);
    > > + return EEXIST;
    > > + }
    > > +
    > > + list_add_tail_rcu(&mqp->list, &mcast->qp_list);
    >
    > Ditto...
    >
    > > + spin_unlock_irqrestore(&mcast_lock, flags);
    > > +
    > > + atomic_inc(&mcast->refcount);
    > > + rb_link_node(&mcast->rb_node, pn, n);
    > > + rb_insert_color(&mcast->rb_node, &mcast_tree);
    > > +
    > > + spin_unlock_irqrestore(&mcast_lock, flags);
    > > +
    > > + return 0;
    > > +}

    --
    Ralph Campbell <ralphc@pathscale.com>

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