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    SubjectRe: Question about usage of RCU in the input layer
    Dmitry Torokhov a écrit :
    > On Thursday 19 March 2009 20:20:32 Arjan van de Ven wrote:
    >>> I don't claim to understand the code in question, so it is entirely
    >>> possible that the following is irrelevant. But one other reason for
    >>> synchronize_rcu() is:
    >>> 1. Make change.
    >>> 2. synchronize_rcu()
    >>> 3. Now you are guaranteed that all CPUs/tasks/whatever
    >>> currently running either are not messing with you on the one hand, or
    >>> have seen the change on the other.
    >> ok so this is for the case where someone is already iterating the list.
    >> I don't see anything in the code that assumes this..
    > This is something that input core guarantees to its users: by the time
    > input core calls hander->start() or, in its absence, by the time
    > input_register_handle() returns, events from input drivers will be
    > passed into the handle being registered, i.e. the presence of the
    > new item in the list is noticed by all CPUs.
    > Now, it is possible to stop using RCU primitives in the input core
    > but I think that you'd want to figure out why synchronize_rcu()
    > takes so long first, otheriwse you may find another "abuser"
    > down the road.

    If a cpu does a loop, it nearly impossible that synchronize_rcu() can
    be fast.

    We had same problem in ksoftirqd(), where I had to add a call
    to rcu_qsctr_inc() to unblock other threads blocked in synchronize_rcu();a=commit;h=64ca5ab913f1594ef316556e65f5eae63ff50cee

    If a driver does a loop with no call to scheduler, it might have same problem

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