Re: [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for simple self-deadlock

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Feb 01, 2012 at 11:41:21AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> It is illegal to have a grace period within a same-flavor RCU read-side
> critical section, so this commit adds lockdep-RCU checks to splat when
> such abuse is encountered.  This commit does not detect more elaborate
> RCU deadlock situations.  These situations might be a job for lockdep
> enhancements.

Since doing so also violates the prohibition on blocking within an RCU
read-side critical section, wouldn't it suffice to call might_sleep() or
equivalent, which also detects other problems?  (Obviously this doesn't
apply to SRCU, but it applies to the other variants of RCU.)

> --- a/kernel/rcutiny.c
> +++ b/kernel/rcutiny.c
> @@ -319,6 +319,9 @@ static void rcu_process_callbacks(struct softirq_action *unused)
>   */
>  void synchronize_sched(void)
>  {
> +	rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
> +			   "Illegal grace period in RCU read-side "
> +			   "critical section");

This message doesn't seem entirely obvious to me.  A grace period didn't
occur; a synchronize call did, which tried to request a grace period
that can never happen.

> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1816,6 +1816,9 @@ EXPORT_SYMBOL_GPL(call_rcu_bh);
>   */
>  void synchronize_sched(void)
>  {
> +	rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
> +			   "Illegal synchronize_sched() in RCU-sched "
> +			   "read-side critical section");
>  	if (rcu_blocking_is_gp())
>  		return;
>  	wait_rcu_gp(call_rcu_sched);
> @@ -1833,6 +1836,9 @@ EXPORT_SYMBOL_GPL(synchronize_sched);
>   */
>  void synchronize_rcu_bh(void)
>  {
> +	rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
> +			   "Illegal synchronize_sched() in RCU-bh "
> +			   "read-side critical section");

Copy-paste problem here: this should say synchronize_sched_bh.  (Or
perhaps it should say __func__. :) )

> --- a/kernel/srcu.c
> +++ b/kernel/srcu.c
> @@ -172,6 +172,10 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
>  {
>  	int idx;
>  
> +	rcu_lockdep_assert(!lock_is_held(&sp->dep_map),
> +			   "Illegal SRCU grace period in same-type "
> +			   "SRCU read-side critical section");

Same issue with the message: a grace period didn't occur, and it never
will; a call to synchronize_srcu requesting a grace period occurred.

- Josh Triplett