Messages in this thread | | | Date | Tue, 3 Nov 2020 11:15:43 +0100 | From | Jan Kara <> | Subject | Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion") |
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On Mon 02-11-20 17:58:54, Filipe Manana wrote: > > > 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?
BTW, I've just hit the same deadlock issue with ext4 on generic/390 so I confirm this isn't btrfs specific issue (as we already knew from the analysis but still it's good to have that confirmed).
Honza -- Jan Kara <jack@suse.com> SUSE Labs, CR
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