On Thu, Nov 19, 2020 at 01:35:12PM -0800, Paul E. McKenney wrote: > On Thu, Nov 19, 2020 at 08:38:19PM +0100, Marco Elver wrote: > > On Thu, Nov 19, 2020 at 10:48AM -0800, Paul E. McKenney wrote: > > > On Thu, Nov 19, 2020 at 06:02:59PM +0100, Marco Elver wrote: > > [ . . . ] > > > > > I can try bisection again, or reverting some commits that might be > > > > suspicious? But we'd need some selection of suspicious commits. > > > > > > The report claims that one of the rcu_node ->lock fields is held > > > with interrupts enabled, which would indeed be bad. Except that all > > > of the stack traces that it shows have these locks held within the > > > scheduling-clock interrupt handler. Now with the "rcu: Don't invoke > > > try_invoke_on_locked_down_task() with irqs disabled" but without the > > > "sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled" > > > commit, I understand why. With both, I don't see how this happens. > > > > I'm at a loss, but happy to keep bisecting and trying patches. I'm also > > considering: > > > > Is it the compiler? Probably not, I tried 2 versions of GCC. > > > > Can we trust lockdep to precisely know IRQ state? I know there's > > been some recent work around this, but hopefully we're not > > affected here? > > > > Is QEMU buggy? > > > > > At this point, I am reduced to adding lockdep_assert_irqs_disabled() > > > calls at various points in that code, as shown in the patch below. > > > > > > At this point, I would guess that your first priority would be the > > > initial bug rather than this following issue, but you never know, this > > > might well help diagnose the initial bug. > > > > I don't mind either way. I'm worried deadlocking the whole system might > > be worse. > > Here is another set of lockdep_assert_irqs_disabled() calls on the > off-chance that they actually find something. FWIW, arm64 is known broken wrt lockdep and irq tracing atm. Mark has been looking at that and I think he is close to having something workable. Mark -- is there anything Marco and Paul can try out? Will
