Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper whendumping connections

From: Eric Dumazet
Date: Fri Apr 26 2013 - 11:59:17 EST

On Fri, 2013-04-26 at 08:45 -0700, Paul E. McKenney wrote:

> I have done some crude coccinelle patterns in the past, but they are
> subject to false positives (from when you transfer the pointer from
> RCU protection to reference-count protection) and also false negatives
> (when you atomically increment some statistic unrelated to protection).
> I could imagine maintaining a per-thread count of the number of outermost
> RCU read-side critical sections at runtime, and then associating that
> counter with a given pointer at rcu_dereference() time, but this would
> require either compiler magic or an API for using a pointer returned
> by rcu_dereference(). This API could in theory be enforced by sparse.
> Dhaval Giani might have some ideas as well, adding him to CC.

We had this fix the otherday, because tcp prequeue code hit this check :

static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
/* If refdst was not refcounted, check we still are in a
* rcu_read_lock section
WARN_ON((skb->_skb_refdst & SKB_DST_NOREF) &&
!rcu_read_lock_held() &&
return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);

( )

Problem is the rcu protected pointer was escaping the rcu lock and was
then used in another thread.

What would be cool (but expensive maybe) , would be to get a cookie from
rcu_read_lock(), and check the cookie at rcu_dereference(). These
cookies would have system wide scope to catch any kind of errors.

Because a per thread counter would not catch following problem :


ptr = rcu_dereference(x);
if (!ptr)
return NULL;


/* no reload of x, ptr might be now stale/freed */
if (ptr->field) { ... }

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