netdev - Re: [PATCH net-next] neigh: Add missing rcu_assign

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Message-ID: <1432808028.24345.5.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 28 May 2015 03:13:48 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ying Xue <ying.xue@...driver.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] neigh: Add missing rcu_assign_pointer

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 <ying.xue@...driver.com>
> ---
>  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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