Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion")

[David Sterba <dsterba@suse.cz>]

On Mon, Oct 26, 2020 at 01:55:24PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 26, 2020 at 11:56:03AM +0000, Filipe Manana wrote:
> > > That smells like the same issue reported here:
> > > 
> > >   https://lkml.kernel.org/r/20201022111700.GZ2651@hirez.programming.kicks-ass.net
> > > 
> > > Make sure you have commit:
> > > 
> > >   f8e48a3dca06 ("lockdep: Fix preemption WARN for spurious IRQ-enable")
> > > 
> > > (in Linus' tree by now) and do you have CONFIG_DEBUG_PREEMPT enabled?
> > 
> > Yes, CONFIG_DEBUG_PREEMPT is enabled.
> 
> Bummer :/

My builds don't have that enabled (CONFIG_PREEMPT_NONE=y) but I still
see the warning (same scenario as for Filipe). That is with today's
master branch + your fix from locking/urgent.

> > I'll try with that commit and let you know, however it's gonna take a
> > few hours to build a kernel and run all fstests (on that test box it
> > takes over 3 hours) to confirm that fixes the issue.
> 
> *ouch*, 3 hours is painful. How long to make it sick with the current
> kernel? quicker I would hope?
> 
> > Thanks for the quick reply!
> 
> Anyway, I don't think that commit can actually explain the issue :/
> 
> The false positive on lockdep_assert_held() happens when the recursion
> count is !0, however we _should_ be having IRQs disabled when
> lockdep_recursion > 0, so that should never be observable.
> 
> My hope was that DEBUG_PREEMPT would trigger on one of the
> __this_cpu_{inc,dec}(lockdep_recursion) instance, because that would
> then be a clear violation.

I can start another round (in my case it's more than 4 hours to
reproduce it) with DEBUG_PREEMPT, unless you have something else to
test.