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    SubjectRe: [PATCH 09/23] ns: Introduce ns_idr to be able to iterate all allocated namespaces in the system
    On Thu, Jul 30, 2020 at 03:00:08PM +0300, Kirill Tkhai wrote:
    > This patch introduces a new IDR and functions to add/remove and iterate
    > registered namespaces in the system. It will be used to list namespaces
    > in /proc/namespaces/... in next patches.

    Looks like you could use an XArray for this and it would be fewer lines of
    code.

    >
    > static struct vfsmount *nsfs_mnt;
    > +static DEFINE_SPINLOCK(ns_lock);
    > +static DEFINE_IDR(ns_idr);

    XArray includes its own spinlock.

    > +/*
    > + * Add a newly created ns to ns_idr. The ns must be fully
    > + * initialized since it becomes available for ns_get_next()
    > + * right after we exit this function.
    > + */
    > +int ns_idr_register(struct ns_common *ns)
    > +{
    > + int ret, id = ns->inum - PROC_NS_MIN_INO;
    > +
    > + if (WARN_ON(id < 0))
    > + return -EINVAL;
    > +
    > + idr_preload(GFP_KERNEL);
    > + spin_lock_irq(&ns_lock);
    > + ret = idr_alloc(&ns_idr, ns, id, id + 1, GFP_ATOMIC);
    > + spin_unlock_irq(&ns_lock);
    > + idr_preload_end();
    > + return ret < 0 ? ret : 0;

    This would simply be return xa_insert_irq(...);

    > +}
    > +
    > +/*
    > + * Remove a dead ns from ns_idr. Note, that ns memory must
    > + * be freed not earlier then one RCU grace period after
    > + * this function, since ns_get_next() uses RCU to iterate the IDR.
    > + */
    > +void ns_idr_unregister(struct ns_common *ns)
    > +{
    > + int id = ns->inum - PROC_NS_MIN_INO;
    > + unsigned long flags;
    > +
    > + if (WARN_ON(id < 0))
    > + return;
    > +
    > + spin_lock_irqsave(&ns_lock, flags);
    > + idr_remove(&ns_idr, id);
    > + spin_unlock_irqrestore(&ns_lock, flags);
    > +}

    xa_erase_irqsave();

    > +
    > +/*
    > + * This returns ns with inum greater than @id or NULL.
    > + * @id is updated to refer the ns inum.
    > + */
    > +struct ns_common *ns_get_next(unsigned int *id)
    > +{
    > + struct ns_common *ns;
    > +
    > + if (*id < PROC_NS_MIN_INO - 1)
    > + *id = PROC_NS_MIN_INO - 1;
    > +
    > + *id += 1;
    > + *id -= PROC_NS_MIN_INO;
    > +
    > + rcu_read_lock();
    > + do {
    > + ns = idr_get_next(&ns_idr, id);
    > + if (!ns)
    > + break;

    xa_find_after();

    You'll want a temporary unsigned long to work with ...

    > + if (!refcount_inc_not_zero(&ns->count)) {
    > + ns = NULL;
    > + *id += 1;

    you won't need this increment.

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