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    SubjectRe: [PATCH 08/11] firewire-sbp-target: Add sbp_login.{c,h}
    On Apr 11 Chris Boot wrote:
    > +static void session_check_for_reset(struct sbp_session *sess)
    > +{
    > + bool card_valid = false;
    > +
    > + spin_lock_bh(&sess->lock);
    > +
    > + if (sess->card) {
    > + spin_lock_irq(&sess->card->lock);
    > + card_valid = (sess->card->local_node != NULL);
    > + spin_unlock_irq(&sess->card->lock);
    > +
    > + if (!card_valid) {
    > + fw_card_put(sess->card);
    > + sess->card = NULL;
    > + }
    > + }
    > +
    > + if (!card_valid || (sess->generation != sess->card->generation)) {
    > + pr_info("Waiting for reconnect from node: %016llx\n",
    > + sess->guid);
    > +
    > + sess->node_id = -1;
    > + sess->reconnect_expires = get_jiffies_64() +
    > + ((sess->reconnect_hold + 1) * HZ);
    > + }
    > +
    > + spin_unlock_bh(&sess->lock);
    > +}

    The card->local_node != NULL test by itself is atomic, it does not benefit
    from being wrapped by card->lock acquisition.

    Well, OK, the lock effectively forces the compiler to determine the value
    of card_valid only once. If the lock weren't there I guess the compiler
    might feel entitled to reload card->local_node in the second !card_valid
    test. But even if you lose ACCESS_ONCE behavior by removing the card->lock
    acquisition, I can't see how that could be detrimental relative to the
    current code.

    I am wondering on the other hand if there isn't actually a dependency
    between this local_node test and something else, e.g. the generation test.
    I.e. might the locking be incomplete? Not sure about that. I think I
    rather want to look at that again when I received the code through
    mainline.

    BTW "card_valid" sounds rather generic; maybe call it "topology_valid"?
    Either way, it can turn valid or invalid any time when firewire-core gets
    to handle a self-ID-complete event.
    --
    Stefan Richter
    -=====-===-- -=-- -===-
    http://arcgraph.de/sr/


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