Messages in this thread | | | From | Anatoly Pugachev <> | Date | Tue, 27 Oct 2020 12:49:10 +0300 | Subject | Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion") |
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On Mon, Oct 26, 2020 at 6:23 PM Peter Zijlstra <peterz@infradead.org> 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 :/ > > 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 ...
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