Messages in this thread | | | Date | Thu, 2 Feb 2012 08:20:17 -0800 | From | "Paul E. McKenney" <> | 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 04:55:54PM -0800, Josh Triplett wrote: > 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.)
Yes, but...
The advantage of the lockdep-RCU splat is that it gives you a better hint as to where the RCU read-side critical section was entered, which is very helpful when tracking these down, especially when they are intermittent.
On of the downsides of the Linux kernel community being more RCU-savvy is that the errors they now tend to commit are more complex. ;-)
And yes, I should also well check for the other variants of RCU read-side critical section (other than RCU). Done.
I also glued the strings together to promote grepability as you suggest later. (But I leave it to you to get checkpatch.pl upgraded -- it currently warns about long lines, but not about strings split across lines.)
> > --- 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.
I suppose I might as well make it consistent with the other messages. ;-)
> > --- 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__. :) )
Fixed, but will pass on __func__ for the moment. Cool though it might be to exercise varargs. ;-)
> > --- 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.
Good catch, fixed!
Thanx, Paul
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