Messages in this thread | | | Subject | Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion") | From | Filipe Manana <> | Date | Mon, 2 Nov 2020 17:58:54 +0000 |
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On 26/10/20 15:22, Peter Zijlstra 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@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 :/ >> >>> 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 :/
Hi Peter,
I still get the same issue with 5.10-rc2. Is there any non-merged patch I should try, or anything I can help with?
Thanks.
> > Anatoly, could you try linus+tip/locking/urgent and the below on your > Sparc, please? > > --- > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c > index 3e99dfef8408..a3041463e42d 100644 > --- a/kernel/locking/lockdep.c > +++ b/kernel/locking/lockdep.c > @@ -84,7 +84,7 @@ static inline bool lockdep_enabled(void) > if (!debug_locks) > return false; > > - if (raw_cpu_read(lockdep_recursion)) > + if (this_cpu_read(lockdep_recursion)) > return false; > > if (current->lockdep_recursion) >
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