Re: [PATCH tip/core/rcu 2/3] rcu: add debug check for too many rcu_read_unlock()

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Jan 04, 2010 at 04:04:01PM -0800, Paul E. McKenney wrote:
> From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> TREE_PREEMPT_RCU maintains an rcu_read_lock_nesting counter in the
> task structure, which happens to be a signed int.  So this patch adds a
> check for this counter being negative at the end of __rcu_read_unlock().
> This check is under CONFIG_PROVE_LOCKING, so can be thought of as being
> part of lockdep.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/rcutree_plugin.h |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index f11ebd4..e77cdf3 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -304,6 +304,9 @@ void __rcu_read_unlock(void)
>  	if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
>  	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
>  		rcu_read_unlock_special(t);
> +#ifdef CONFIG_PROVE_LOCKING
> +	WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
> +#endif /* #ifdef CONFIG_PROVE_LOCKING */
>  }
>  EXPORT_SYMBOL_GPL(__rcu_read_unlock);

Given that you *already* need to access t->rcu_read_lock_nesting here,
why not just do the test all the time?  Ideally you could access
t->rcu_read_lock_nesting once, decrement it, and test for both 0 and
negative.

- Josh Triplett