On Thu, Nov 19, 2020 at 01:35PM -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. > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit bcca5277df3f24db15e15ccc8b05ecf346d05169 > Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > Date: Thu Nov 19 13:30:33 2020 -0800 > > rcu: Add lockdep_assert_irqs_disabled() to raw_spin_unlock_rcu_node() macros None of those triggered either. I found that disabling ftrace for some of kernel/rcu (see below) solved the stalls (and any mention of deadlocks as a side-effect I assume), resulting in successful boot. Does that provide any additional clues? I tried to narrow it down to 1-2 files, but that doesn't seem to work. Thanks, -- Marco ------ >8 ------ diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile index 0cfb009a99b9..678b4b094f94 100644 --- a/kernel/rcu/Makefile +++ b/kernel/rcu/Makefile @@ -3,6 +3,13 @@ # and is generally not a function of system call inputs. KCOV_INSTRUMENT := n +ifdef CONFIG_FUNCTION_TRACER +CFLAGS_REMOVE_update.o = $(CC_FLAGS_FTRACE) +CFLAGS_REMOVE_sync.o = $(CC_FLAGS_FTRACE) +CFLAGS_REMOVE_srcutree.o = $(CC_FLAGS_FTRACE) +CFLAGS_REMOVE_tree.o = $(CC_FLAGS_FTRACE) +endif + ifeq ($(CONFIG_KCSAN),y) KBUILD_CFLAGS += -g -fno-omit-frame-pointer endif
