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SubjectRe: [PATCH 09/23] ns: Introduce ns_idr to be able to iterate all allocated namespaces in the system
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On 30.07.2020 15:23, Matthew Wilcox wrote:
> 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();

static inline void *xa_erase_irqsave(struct xarray *xa, unsigned long index)
{
unsigned long flags;
void *entry;

xa_lock_irqsave(xa, flags);
entry = __xa_erase(xa, index);
xa_unlock_irqrestore(xa, flags);

return entry;
}

>> +
>> +/*
>> + * 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.

Why? I don't see a way xarray allows to avoid this.

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