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    SubjectRe: [PATCH tip/core/rcu 02/26] rcu: New rcu_user_enter_irq() and rcu_user_exit_irq() APIs
    On Thu, Aug 30, 2012 at 02:05:19PM -0700, Paul E. McKenney wrote:
    > From: Frederic Weisbecker <fweisbec@gmail.com>
    >
    > In some cases, it is necessary to enter or exit userspace-RCU-idle mode
    > from an interrupt handler, for example, if some other CPU sends this
    > CPU a resched IPI. In this case, the current CPU would enter the IPI
    > handler in userspace-RCU-idle mode, but would need to exit the IPI handler
    > after having exited that mode.
    >
    > To allow this to work, this commit adds two new APIs to TREE_RCU:
    >
    > - rcu_user_enter_irq(). This must be called from an interrupt between
    > rcu_irq_enter() and rcu_irq_exit(). After the irq calls rcu_irq_exit(),
    > the irq handler will return into an RCU extended quiescent state.
    > In theory, this interrupt is never a nested interrupt, but in practice
    > it might interrupt softirq, which looks to RCU like a nested interrupt.
    >
    > - rcu_user_exit_irq(). This must be called from a non-nesting
    > interrupt, interrupting an RCU extended quiescent state, also
    > between rcu_irq_enter() and rcu_irq_exit(). After the irq calls
    > rcu_irq_exit(), the irq handler will return in an RCU non-quiescent
    > state.

    These names seem a bit confusing. From the descriptions, it sounds like
    you don't always need to pair them; rcu_irq_exit() will return to a
    non-quiescent state, unless you call rcu_user_enter_irq and *don't* call
    rcu_user_exit_irq. Did I get that semantic right?

    Given that, the "enter" and "exit" names seem confusing. This seems
    more like a flag you can set and clear, rather than a delimited region
    as suggested by an enter/exit pair.

    How about something vaguely like rcu_user_irq_set_eqs and
    rcu_user_irq_clear_eqs?

    - Josh Triplett


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