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

[Anatoly Pugachev]

On Mon, Oct 26, 2020 at 6:23 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > > >
> > > > 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 :/
> >
> > > 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.
> >
> > And you're seeing this on x86, right?
> >
> > Let me puzzle moar..
>
> So I might have an explanation for the Sparc64 fail, but that can't
> explain x86 :/
>
> I initially thought raw_cpu_read() was OK, since if it is !0 we have
> IRQs disabled and can't get migrated, so if we get migrated both CPUs
> must have 0 and it doesn't matter which 0 we read.
>
> And while that is true; it isn't the whole store, on pretty much all
> architectures (except x86) this can result in computing the address for
> one CPU, getting migrated, the old CPU continuing execution with another
> task (possibly setting recursion) and then the new CPU reading the value
> of the old CPU, which is no longer 0.
>
> I already fixed a bunch of that in:
>
>   baffd723e44d ("lockdep: Revert "lockdep: Use raw_cpu_*() for per-cpu variables"")
>
> but clearly this one got crossed.
>
> Still, that leaves me puzzled over you seeing this on x86 :/
>
> Anatoly, could you try linus+tip/locking/urgent and the below on your
> Sparc, please?

Peter,
let me test first. Thanks.

PS: sorry for the delay, a weekend and got ill a bit ...