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commit 09a26e83 upstream.

Guillaume Morin reported hitting the following WARNING followed by GPF or
NULL pointer deference either in cgroups_destroy or in the kill_css path.:

    percpu ref (css_release) <= 0 (-1) after switching to atomic
    WARNING: CPU: 23 PID: 130 at lib/percpu-refcount.c:196 percpu_ref_switch_to_atomic_rcu+0x127/0x130
    CPU: 23 PID: 130 Comm: ksoftirqd/23 Kdump: loaded Tainted: G           O      5.10.60 #1
    RIP: 0010:percpu_ref_switch_to_atomic_rcu+0x127/0x130
    Call Trace:
       rcu_core+0x30f/0x530
       rcu_core_si+0xe/0x10
       __do_softirq+0x103/0x2a2
       run_ksoftirqd+0x2b/0x40
       smpboot_thread_fn+0x11a/0x170
       kthread+0x10a/0x140
       ret_from_fork+0x22/0x30

Upon further examination, it was discovered that the css structure was
associated with hugetlb reservations.

For private hugetlb mappings the vma points to a reserve map that
contains a pointer to the cs...