On Fri, Apr 26, 2013 at 10:03:13AM +0200, Peter Zijlstra wrote:
> On Fri, Apr 26, 2013 at 10:45:08AM +0900, Simon Horman wrote:
>
> > @@ -975,8 +975,7 @@ static void *ip_vs_conn_array(struct seq_file *seq,
> > loff_t pos)
> > return cp;
> > }
> > }
> > - rcu_read_unlock();
> > - rcu_read_lock();
> > + cond_resched_rcu_lock();
> > }
>
>
> While I agree with the sentiment I do find it a somewhat dangerous construct
> in
> that it might become far too easy to keep an RCU reference over this break and
> thus violate the RCU premise.
>
> Is there anything that can detect this? Sparse / cocinelle / smatch? If so it
> would be great to add this to these checkers.
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.
Thanx, Paul
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