Q: lockdep_assert_held_read() after downgrade_write()

[J. R. Okajima]

Peter Zijlstra,

May I ask you a question?
v4.10-rc1 got a commit
        f831948 2016-11-30 locking/lockdep: Provide a type check for 
lock_is_held
I've tested a little and lockdep splat a stack trace.

{
        DECLARE_RWSEM(rw);
        static struct lock_class_key key;
        lockdep_set_class(&rw, &key);

        down_read(&rw);
        lockdep_assert_held_read(&rw);
        up_read(&rw);

        down_write(&rw);
        lockdep_assert_held_exclusive(&rw);
        up_write(&rw);

        downgrade_write(&rw);
        lockdep_assert_held_read(&rw);  <-- here
        up_read(&rw);
}

I was expecting that lockdep_assert_held_read() splat nothing after
downgrade_write(). Is this warning an intentional behaviour?

Also the final up_read() gives me a warning too. It is produced at
        lockdep.c:3514:lock_release(): DEBUG_LOCKS_WARN_ON(depth <= 0)

As an additional information, I increased some lockdep constants.
Do you think this is related?

include/linux/lockdep.h
+#define MAX_LOCKDEP_SUBCLASSES         (8UL + 4)
+#define MAX_LOCKDEP_KEYS_BITS          (13 + 3)

kernel/locking/lockdep_internals.h
+#define MAX_LOCKDEP_ENTRIES    (32768UL << 5)
+#define MAX_LOCKDEP_CHAINS_BITS        (16 + 5)
+#define MAX_STACK_TRACE_ENTRIES        (524288UL << 5)


J. R. Okajima