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    SubjectRe: linux-next: stall warnings and deadlock on Arm64 (was: [PATCH] kfence: Avoid stalling...)
    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@kernel.org>
    Date: Thu Nov 19 13:30:33 2020 -0800

    rcu: Add lockdep_assert_irqs_disabled() to raw_spin_unlock_rcu_node() macros

    This commit adds a lockdep_assert_irqs_disabled() call to the
    helper macros that release the rcu_node structure's ->lock, namely
    to raw_spin_unlock_rcu_node(), raw_spin_unlock_irq_rcu_node() and
    raw_spin_unlock_irqrestore_rcu_node(). The point of this is to help track
    down a situation where lockdep appears to be insisting that interrupts
    are enabled while holding an rcu_node structure's ->lock.

    Link: https://lore.kernel.org/lkml/20201111133813.GA81547@elver.google.com/
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

    diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
    index 59ef1ae..bf0827d 100644
    --- a/kernel/rcu/rcu.h
    +++ b/kernel/rcu/rcu.h
    @@ -378,7 +378,11 @@ do { \
    smp_mb__after_unlock_lock(); \
    } while (0)

    -#define raw_spin_unlock_rcu_node(p) raw_spin_unlock(&ACCESS_PRIVATE(p, lock))
    +#define raw_spin_unlock_rcu_node(p) \
    +do { \
    + lockdep_assert_irqs_disabled(); \
    + raw_spin_unlock(&ACCESS_PRIVATE(p, lock)); \
    +} while (0)

    #define raw_spin_lock_irq_rcu_node(p) \
    do { \
    @@ -387,7 +391,10 @@ do { \
    } while (0)

    #define raw_spin_unlock_irq_rcu_node(p) \
    - raw_spin_unlock_irq(&ACCESS_PRIVATE(p, lock))
    +do { \
    + lockdep_assert_irqs_disabled(); \
    + raw_spin_unlock_irq(&ACCESS_PRIVATE(p, lock)); \
    +} while (0)

    #define raw_spin_lock_irqsave_rcu_node(p, flags) \
    do { \
    @@ -396,7 +403,10 @@ do { \
    } while (0)

    #define raw_spin_unlock_irqrestore_rcu_node(p, flags) \
    - raw_spin_unlock_irqrestore(&ACCESS_PRIVATE(p, lock), flags)
    +do { \
    + lockdep_assert_irqs_disabled(); \
    + raw_spin_unlock_irqrestore(&ACCESS_PRIVATE(p, lock), flags); \
    +} while (0)

    #define raw_spin_trylock_rcu_node(p) \
    ({ \
    \
     
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