Messages in this thread | | | Date | Wed, 5 May 2010 16:05:57 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 01/48] rcu: optionally leave lockdep enabled after RCU lockdep splat |
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On Wed, May 05, 2010 at 06:46:41PM -0400, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > From: Lai Jiangshan <laijs@cn.fujitsu.com> > > > > There is no need to disable lockdep after an RCU lockdep splat, > > so remove the debug_lockdeps_off() from lockdep_rcu_dereference(). > > To avoid repeated lockdep splats, use a static variable in the inlined > > rcu_dereference_check() and rcu_dereference_protected() macros so that > > a given instance splats only once, but so that multiple instances can > > be detected per boot. > > > > This is controlled by a new config variable CONFIG_PROVE_RCU_REPEATEDLY, > > which is disabled by default. This provides the normal lockdep behavior > > by default, but permits people who want to find multiple RCU-lockdep > > splats per boot to easily do so. > > I'll play the devil's advocate here. (just because that's so much fun) > ;-) > > If we look at: > > include/linux/debug_locks.h: > > static inline int __debug_locks_off(void) > { > return xchg(&debug_locks, 0); > } > > We see that all code following a call to "debug_locks_off()" can assume > that it cannot possibly run concurrently with other code following > "debug_locks_off()". Now, I'm not saying that the code we currently have > will necessarily break, but I think it is worth asking if there is any > assumption in lockdep (or RCU lockdep more specifically) about mutual > exclusion after debug_locks_off() ? > > Because if there is, then this patch is definitely breaking something by > not protecting lockdep against multiple concurrent executions.
So what in lockdep_rcu_dereference() needs to be protected from concurrent execution? All that happens beyond that point is a bunch of printk()s, printing the locks held by this task, and dumping this task's stack.
Thanx, Paul
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