Re: [PATCH tip/core/rcu 02/26] rcu: New rcu_user_enter_irq() and rcu_user_exit_irq() APIs

Frederic Weisbecker
2012-08-31T19:54:42Z

2012/8/31 Josh Triplett <josh@joshtriplett.org>:
> 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?

Yeah. They indeed don't always need to be paired. We can enter into
user (from rcu POV) with rcu_user_enter_irq() and exit user with
rcu_user_exit().

It's just a matter of context: from where do we set/unset RCU as in
user mode: irq or not. The only thing that is paired is the fact we
enter/exit that RCU user mode. There are just different APIs to do so.

> 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?

I'd rather suggest rcu_user_enter_after_irq and
rcu_user_exit_after_irq. It describes precisely what it does.
>
> - Josh Triplett