rcu: optionally leave lockdep enabled after RCU lockdep splat
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From: Paul E. McKenney on 6 May 2010 19:10 On Wed, May 05, 2010 at 10:05:32PM -0400, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck(a)linux.vnet.ibm.com) wrote: > > On Wed, May 05, 2010 at 07:24:57PM -0400, Mathieu Desnoyers wrote: > > > * Paul E. McKenney (paulmck(a)linux.vnet.ibm.com) wrote: > > > > On Wed, May 05, 2010 at 06:46:41PM -0400, Mathieu Desnoyers wrote: > > > > > * Paul E. McKenney (paulmck(a)linux.vnet.ibm.com) wrote: > > > > > > From: Lai Jiangshan <laijs(a)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 > > > > > > I agree with you that printing the current task information should be safe. > > > However, I am not sure that concurrent updates to the lock_class while printk() > > > is showing its information will end up doing what we expect it to do. > > > > > > It could be acceptable to have unreliable information in these rare cases, but > > > the important thing would be to ensure that the kernel does not OOPS. > > > > But any races other than the printk()s can already happen as follows: > > > > o CPU 0 needs to update some information about the lock. It > > checks debug_locks and finds that it is non-zero. > > > > o CPU 1 detects a deadlock, and invokes __debug_locks_off(), > > which atomically sets debug_locks to zero. > > > > o CPU 1 then proceeds to printk() information that CPU 0 > > is concurrently modifying. Which looks to be OK in any case. > > > > Or is there some other race that cannot already happen that I am > > introducing? > > Nope, I don't think so. Although it's probably worth putting a comment in > lockdep_rcu_dereference() to state that lockdep can be used by multiple > concurrent instances here, just in case someone ever consider adding code > to this splat handler thinking lockdep is always only used by a single "splat" > at a time. Done! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |