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    SubjectRe: INFO: possible circular locking dependency at cleanup_workqueue_thread
    On 05/20, Peter Zijlstra wrote:
    >
    > > > =======================================================
    > > > [ INFO: possible circular locking dependency detected ]
    > > > 2.6.30-rc5-00097-gd665355 #59
    > > > -------------------------------------------------------
    > > > pm-suspend/12129 is trying to acquire lock:
    > > > (events){+.+.+.}, at: [<ffffffff80259496>] cleanup_workqueue_thread+0x26/0xd0
    > > >
    > > > but task is already holding lock:
    > > > (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff80246e57>]
    > > > cpu_maps_update_begin+0x17/0x20
    > > >
    > > > which lock already depends on the new lock.
    > > >
    > > >
    > > > the existing dependency chain (in reverse order) is:
    > > >
    > > > -> #5 (cpu_add_remove_lock){+.+.+.}:
    > > > [<ffffffff80271a64>] __lock_acquire+0xc64/0x10a0
    > > > [<ffffffff80271f38>] lock_acquire+0x98/0x140
    > > > [<ffffffff8054e78c>] __mutex_lock_common+0x4c/0x3b0
    > > > [<ffffffff8054ebf6>] mutex_lock_nested+0x46/0x60
    > > > [<ffffffff80246e57>] cpu_maps_update_begin+0x17/0x20
    > > > [<ffffffff80259c33>] __create_workqueue_key+0xc3/0x250
    > > > [<ffffffff80287b20>] stop_machine_create+0x40/0xb0
    > > > [<ffffffff8027a784>] sys_delete_module+0x84/0x270
    > > > [<ffffffff8020c15b>] system_call_fastpath+0x16/0x1b
    > > > [<ffffffffffffffff>] 0xffffffffffffffff
    >
    > Oleg, why does __create_workqueue_key() require cpu_maps_update_begin()?
    > Wouldn't get_online_cpus() be enough to freeze the online cpus?

    Yes, get_online_cpus() pins online CPUs. But CPU_POST_DEAD calls
    cleanup_workqueue_thread() without cpu_hotplug.lock, this means
    that create/destroy can race with cpu_down().

    We can avoid cpu_add_remove_lock, but then we have to add another
    lock to protect workqueues, cpu_populated_map, etc.

    > Breaking the setup_lock -> cpu_add_remove_lock dependency seems
    > sufficient.

    Hmm. What do you mean? Afaics setup_lock -> cpu_add_remove_lock
    is not a problem?

    Oleg.



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