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    SubjectRe: [PATCH 1/2] cpuhotplug: use rw_semaphore for cpu_hotplug
    Oleg Nesterov wrote:
    > On 05/29, Oleg Nesterov wrote:
    >> On 05/29, Andrew Morton wrote:
    >>> On Fri, 29 May 2009 16:29:30 +0800
    >>> Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
    >>>
    >>>> Current get_online_cpus()/put_online_cpus() re-implement
    >>>> a rw_semaphore,
    >>>> so it is converted to a real rw_semaphore in this fix.
    >>>> It simplifies codes, and is good for read.
    >>>> static struct {
    >>>> - struct task_struct *active_writer;
    >>>> - struct mutex lock; /* Synchronizes accesses to refcount, */
    >>>> /*
    >>>> - * Also blocks the new readers during
    >>>> - * an ongoing cpu hotplug operation.
    >>>> + * active_writer makes get_online_cpus()/put_online_cpus() are allowd
    >>>> + * to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
    >>>> + *
    >>>> + * Thus, get_online_cpus()/put_online_cpus() can be called in
    >>>> + * CPU notifiers.
    >>>> */
    >>>> - int refcount;
    >>>> + struct task_struct *active_writer;
    >>>> + struct rw_semaphore rwlock;
    >>>> } cpu_hotplug;
    >> But, afaics, down_write() blocks new readers.
    >>
    >> This means that with this patch get_online_cpus() is not recursive, no?
    >
    > And please note that the current code drops mutex when get_online_cpus()
    > succeeds. With your patch (if I read it correctly) the code under get_()
    > runs with cpu_hotplug->rwlock held for reading. I'm afraid this creates
    > the new possibilities for deadlocks.
    >

    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.

    Lai.



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