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    SubjectRe: [PATCH] arp_queue: serializing unlink + kfree_skb
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    On Thu, Feb 03, 2005 at 03:08:21PM -0800, David S. Miller wrote:
    >
    > Ok, here goes nothing. Can someone run with this? It should
    > be rather complete, and require only minor editorial work.

    Thanks. It's a very nice piece of work.

    > A missing memory barrier in the cases where they are required
    > by the atomic_t implementation above can have disasterous
    > results. Here is an example, which follows a pattern occuring
    > frequently in the Linux kernel. It is the use of atomic counters
    > to implement reference counting, and it works such that once
    > the counter falls to zero it can be guarenteed that no other
    > entity can be accessing the object. Observe:
    >
    > list_del(&obj->list);
    > if (atomic_dec_and_test(&obj->ref_count))
    > kfree(obj);
    >
    > Here, the list (say it is some linked list on which object
    > searches are performed) creates the reference to the object.
    > The insertion code probably looks something like this:
    >
    > atomic_inc(&obj->ref_count);
    > list_add(&obj->list, &global_obj_list);

    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.

    > And searches look something like:
    >
    > for_each(obj, &global_obj_list) {
    > if (key_compare(obj->key, key)) {
    > atomic_inc(&obj->ref_count);
    > return obj;
    > }
    > }
    > return NULL;

    Locking or RCU is definitely needed here.

    > the object is still visible for lookup on the linked list. So
    > we'd get a bogus sequence like this:
    >
    > cpu 0 cpu 1
    > list_del(&obj->list);
    > ... visibility delayed ...
    > lookup and find obj on
    > global_obj_list

    The visibility only needs to be delayed up until this point for
    the crash to occur.

    > atomic_dec_and_test();
    > obj refcount hits zero, this
    > is visible globally
    > atomic_inc()
    > obj refcount incremented
    > to one
    > list_del() becomes visible
    >
    > kfree(obj);
    > obj is now freed up memory
    > --> CRASH
    >
    > With the memory barrier semantics required of the atomic_t
    > operations which return values, the above sequence of memory
    > visibility can never happen.

    So this isn't exactly correct.

    Cheers,
    --
    Visit Openswan at http://www.openswan.org/
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