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


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