On Thu, 2015-05-28 at 16:28 +0800, Ying Xue wrote:
> Commit e4c4e448cf55 ("neigh: Convert garbage collection from softirq
> to workqueue") misses to use rcu_assign_pointer() macro to assign a
> RCU-protected pointer.
>
> Signed-off-by: Ying Xue <[email protected]>
> ---
> net/core/neighbour.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 3a74df7..aaad3a5 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -783,7 +783,8 @@ static void neigh_periodic_work(struct work_struct *work)
> if (atomic_read(&n->refcnt) == 1 &&
> (state == NUD_FAILED ||
> time_after(jiffies, n->used + NEIGH_VAR(n->parms,
> GC_STALETIME)))) {
> - *np = n->next;
> + rcu_assign_pointer(*np,
> rcu_dereference_protected(n->next,
> +
> lockdep_is_held(&tbl->lock)));
> n->dead = 1;
> write_unlock(&n->lock);
> neigh_cleanup_and_release(n);
This patch is not needed.
You really should read Documentation/RCU , because it looks like you are
quite confused.
When we remove an element from a RCU protected list, all the objects in
the chain are already ready to be caught by rcu readers.
Therefore, no additional memory barrier is needed before doing *np =
n->next;
Please do not add spurious memory barriers. Like atomic operations, we
want all of them being required and possibly documented.
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