Messages in this thread | | | Date | Fri, 3 Feb 2012 10:05:27 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for simple self-deadlock |
| |
On Fri, Feb 03, 2012 at 01:04:49AM -0800, Josh Triplett wrote: > On Thu, Feb 02, 2012 at 12:42:06PM -0800, Paul E. McKenney wrote: > > On Thu, Feb 02, 2012 at 11:56:38AM -0800, Josh Triplett wrote: > > > On Thu, Feb 02, 2012 at 08:20:17AM -0800, Paul E. McKenney wrote: > > > > 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. > > > > > > Ah, fair enough. > > > > > > > And yes, I should also well check for the other variants of RCU read-side > > > > critical section (other than RCU). Done. > > > > > > Oh? What hadn't you checked for? > > > > Things like synchronize_sched() in rcu_read_lock() critical section > > and vice versa. > > Ouch. Good idea. > > That also suggests another interesting possibility: lockdep could tag > pointers used in the flavor-specific rcu_dereference variants and > pointers used in the call_rcu variants to make sure nobody uses multiple > variants on the same pointer. :) (Assuming we don't want > flavor-specific __rcu_* pointer tags.)
Indeed, the last attempt to produce flavor-specific __rcu_* pointer tags turned into quite a mess. The other issue with it is that it looks like there are reasonable use cases for protecting a given pointer with multiple flavors of RCU. I don't know if any of them have actually made it into mainline, but there have been a number of discussions involving them.
> Speaking of which, could kfree_rcu require its argument to have the > __rcu type annotation? We can't necessarily guarantee that for call_rcu > in all cases, but I think we can for kfree_rcu.
It might make sense -- I have added it to my list of things to think about for RCU.
Thanx, Paul
| |