Re: [tip: locking/core] lockdep: Fix lockdep recursion

[Qian Cai]

On Mon, 2020-10-12 at 11:11 +0800, Boqun Feng wrote:
> Hi,
> 
> On Fri, Oct 09, 2020 at 09:41:24AM -0400, Qian Cai wrote:
> > On Fri, 2020-10-09 at 07:58 +0000, tip-bot2 for Peter Zijlstra wrote:
> > > The following commit has been merged into the locking/core branch of tip:
> > > 
> > > Commit-ID:     4d004099a668c41522242aa146a38cc4eb59cb1e
> > > Gitweb:        
> > > https://git.kernel.org/tip/4d004099a668c41522242aa146a38cc4eb59cb1e
> > > Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > AuthorDate:    Fri, 02 Oct 2020 11:04:21 +02:00
> > > Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
> > > CommitterDate: Fri, 09 Oct 2020 08:53:30 +02:00
> > > 
> > > lockdep: Fix lockdep recursion
> > > 
> > > Steve reported that lockdep_assert*irq*(), when nested inside lockdep
> > > itself, will trigger a false-positive.
> > > 
> > > One example is the stack-trace code, as called from inside lockdep,
> > > triggering tracing, which in turn calls RCU, which then uses
> > > lockdep_assert_irqs_disabled().
> > > 
> > > Fixes: a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per-
> > > cpu
> > > variables")
> > > Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> > 
> > Reverting this linux-next commit fixed booting RCU-list warnings everywhere.
> > 
> 
> I think this happened because in this commit debug_lockdep_rcu_enabled()
> didn't adopt to the change that made lockdep_recursion a percpu
> variable?
> 
> Qian, mind to try the following?

Yes, it works fine.

> 
> Although, arguably the problem still exists, i.e. we still have an RCU
> read-side critical section inside lock_acquire(), which may be called on
> a yet-to-online CPU, which RCU doesn't watch. I think this used to be OK
> because we don't "free" anything from lockdep, IOW, there is no
> synchronize_rcu() or call_rcu() that _needs_ to wait for the RCU
> read-side critical sections inside lockdep. But now we lock class
> recycling, so it might be a problem.
> 
> That said, currently validate_chain() and lock class recycling are
> mutually excluded via graph_lock, so we are safe for this one ;-)
> 
> ----------->8
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 39334d2d2b37..35d9bab65b75 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -275,8 +275,8 @@ EXPORT_SYMBOL_GPL(rcu_callback_map);
>  
>  noinstr int notrace debug_lockdep_rcu_enabled(void)
>  {
> -	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
> -	       current->lockdep_recursion == 0;
> +	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE &&
> +	       __lockdep_enabled;
>  }
>  EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);
>