On Tue, Nov 27, 2012 at 09:01:33AM -0500, Sasha Levin wrote: > On 11/27/2012 08:07 AM, Gleb Natapov wrote: > > Those rcu_irq_enter()/rcu_irq_exit() were introduced by commit > > c5e015d4949aa665 "KVM guest: exit idleness when handling > > KVM_PV_REASON_PAGE_NOT_PRESENT", but now I am starting to question this > > commit. KVM_PV_REASON_PAGE_NOT_PRESENT should not kick cpu out of > > idleness. kvm_async_pf_task_wait() checks that cpu is idle and calls > > halt if it is. After that commit schedule() may be called between > > rcu_irq_enter()/rcu_irq_exit() which is probably illegal. Paul? It is legal to call rcu_irq_enter() and then schedule(). In fact, it turns out that it -has- to be legal, due to some architectures' quaint habit of entering interrupt/exception handlers that they never leave, and possibly vice versa. > otoh, calling schedule() apparently kicks cpu out of idleness now. But if you call rcu_irq_enter() and then schedule(), and if schedule() switches to the idle thread, and if execution proceeds to the point where rcu_idle_enter() is called, then, RCU will quite naturally decide that it is fully idle. At that point, it is illegal to invoke rcu_irq_exit() unless/until you have either: (1) exited the idle loop (as in called rcu_idle_exit()) or (2) taken an interrupt, which will call rcu_irq_enter(). And to think that when I started coding RCU's dyntick-idle funtionality, I was thinking in terms of a simple nesting counter. Silly me! ;-) Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
