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SubjectRe: [PATCH tip/core/rcu 01/48] rcu: optionally leave lockdep enabled after RCU lockdep splat
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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