Re: [PATCH] arp_queue: serializing unlink + kfree_skb

From: David S. Miller (davem_at_davemloft.net)
Date: 02/05/05

  • Next message: Rafael J. Wysocki: "Re: 2.6.11-rc3-mm1: device_resume() hangs on Athlon64" 
    Date:	Fri, 4 Feb 2005 15:48:55 -0800
To: Herbert Xu <herbert@gondor.apana.org.au>

    On Fri, 4 Feb 2005 22:33:05 +1100
    Herbert Xu <herbert@gondor.apana.org.au> wrote:

    > I think you should probably note that some sort of locking or RCU
    > scheme is required to make this safe. As it is the atomic_inc
    > and the list_add can be reordered such that the atomic_inc occurs
    > after the atomic_dec_and_test.
    >
    > Either that or you can modify the example to add an
    > smp_mb__after_atomic_inc(). That'd be a good way to
    > demonstrate its use.

    Yeah, this example is totally bogus. I'll make it match the
    neighbour cache case which started this discussion. Which is
    something like:

    static void obj_list_add(struct obj *obj)
    {
            obj->active = 1;
            list_add(&obj->list);
    }

    static void obj_list_del(struct obj *obj)
    {
            list_del(&obj->list);
            obj->active = 0;
    }

    static void obj_destroy(struct obj *obj)
    {
            BUG_ON(obj->active);
            kfree(obj);
    }

    struct obj *obj_list_peek(struct list_head *head)
    {
            if (!list_empty(head)) {
                    struct obj *obj;

                    obj = list_entry(head->next, struct obj, list);
                    atomic_inc(&obj->refcnt);
                    return obj;
            }
            return NULL;
    }

    void obj_poke(void)
    {
            struct obj *obj;

            spin_lock(&global_list_lock);
            obj = obj_list_peek(&global_list);
            spin_unlock(&global_list_lock);

            if (obj) {
                    obj->ops->poke(obj);
                    if (atomic_dec_and_test(&obj->refcnt))
                            obj_destroy(obj);
            }
    }

    void obj_timeout(struct obj *obj)
    {
            spin_lock(&global_list_lock);
            obj_list_del(obj);
            spin_unlock(&global_list_lock);

            if (atomic_dec_and_test(&obj->refcnt))
                    obj_destroy(obj);
    }

    Something like that. I'll update the atomic_ops.txt
    doc and post and updated version later tonight.
    -
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  • Next message: Rafael J. Wysocki: "Re: 2.6.11-rc3-mm1: device_resume() hangs on Athlon64"

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