DRBD源码分析（四）——activelog

/* We maintain a trivial check sum in our on disk activity log.
 * With that we can ensure correct operation even when the storage
 * device might do a partial (last) sector write while loosing power.
 */
struct __packed al_transaction {
    u32       magic;
    u32       tr_number;
    struct __packed {
        u32 pos;
        u32 extent; } updates[1 + AL_EXTENTS_PT];
    u32       xor_sum;
};

void drbd_al_begin_io(struct drbd_conf *mdev, sector_t sector)
{
    unsigned int enr = (sector >> (AL_EXTENT_SHIFT-9));
    struct lc_element *al_ext;
    struct update_al_work al_work;

    D_ASSERT(atomic_read(&mdev->local_cnt) > 0);

    trace_drbd_actlog(mdev, sector, "al_begin_io");

    wait_event(mdev->al_wait, (al_ext = _al_get(mdev, enr)));

    if (al_ext->lc_number != enr) {
        /* drbd_al_write_transaction(mdev,al_ext,enr);
         * recurses into generic_make_request(), which
         * disallows recursion, bios being serialized on the
         * current->bio_tail list now.
         * we have to delegate updates to the activity log
         * to the worker thread. */
        init_completion(&al_work.event);
        al_work.al_ext = al_ext;
        al_work.enr = enr;
        al_work.old_enr = al_ext->lc_number;
        al_work.w.cb = w_al_write_transaction;
        drbd_queue_work_front(&mdev->data.work, &al_work.w);
        wait_for_completion(&al_work.event);
        dev_err(DEV, "w_al_write_transaction called\n");
        mdev->al_writ_cnt++;

        /*
        DUMPI(al_ext->lc_number);
        DUMPI(mdev->act_log->new_number);
        */
        spin_lock_irq(&mdev->al_lock);
        lc_changed(mdev->act_log, al_ext);
        spin_unlock_irq(&mdev->al_lock);
        wake_up(&mdev->al_wait);
    }
}