[BUG] mm: backing-dev: a possible sleep-in-atomic-context bug in cgwb_create()

[Jia-Ju Bai]

The kernel may sleep with holding a spinlock.
The function call path (from bottom to top) in Linux-4.16.7 is:

[FUNC] schedule
lib/percpu-refcount.c, 222:
        schedule in __percpu_ref_switch_mode
lib/percpu-refcount.c, 339:
        __percpu_ref_switch_mode in percpu_ref_kill_and_confirm
./include/linux/percpu-refcount.h, 127:
        percpu_ref_kill_and_confirm in percpu_ref_kill
mm/backing-dev.c, 545:
        percpu_ref_kill in cgwb_kill
mm/backing-dev.c, 576:
        cgwb_kill in cgwb_create
mm/backing-dev.c, 573:
        _raw_spin_lock_irqsave in cgwb_create

This bug is found by my static analysis tool (DSAC-2) and checked by my
code review.

I do not know how to correctly fix this bug, so I just report them.
Maybe cgwb_kill() should not be called with holding a spinlock.


Best wishes,
Jia-Ju Bai