| Date | Wed, 1 Feb 2012 16:55:54 -0800 | From | Josh Triplett <> | Subject | Re: [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for simple self-deadlock |
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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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