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    SubjectRe: [PATCH 1/2] cpuhotplug: use rw_semaphore for cpu_hotplug
    Lai Jiangshan wrote:
    > down_read()/up_read() can be nested within down_read()/up_read(),
    > so get_online_cpus() is recursive.
    > And thanks to cpu_hotplug.active_writer, get_online_cpus()/put_online_cpus()
    > are allowd to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
    > So cpu_hotplug_begin() DO NOT blocks readers who are in CPU notifiers.

    Lai Jiangshan wrote:
    > The current code drops mutex when get_online_cpus() succeeds, BUT it
    > increases the counter as what down_read() does. I think the current
    > code has the same deadlocks which the down_read()-implement has.
    > Since the current code use mutex + counter to implement a "down_read()",
    > why not use the down_read() directly?
    > And down_read() can be checked by lockdep.

    Ouch, the kernel rw_semaphore is not Read-preference. All what I said
    is garbage. I did miss this, sorry for bothered you all.


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