On Fri, Nov 30, 2012 at 04:36:46PM +0800, Li Zhong wrote: > On Thu, 2012-11-29 at 19:25 +0200, Gleb Natapov wrote: > > On Thu, Nov 29, 2012 at 03:40:05PM +0100, Frederic Weisbecker wrote: > > > 2012/11/29 Li Zhong <zhong@xxxxxxxxxxxxxxxxxx>: > > > > On Wed, 2012-11-28 at 13:55 +0100, Frederic Weisbecker wrote: > > > >> > With rcu_user_exit() at the beginning, now rcu_irq_enter() only protects > > > >> > the cpu idle eqs, but it's not good to call rcu_irq_exit() after the cpu > > > >> > halt and the page ready. > > > >> > > > >> Hmm, why is it not good? > > > > > > > > I think in this case, as cpu halts and waits for page ready, it is > > > > really in idle, and it's better for rcu to think it as rcu idle. But > > > > with rcu_irq_enter(), the rcu would not think this cpu as idle. And the > > > > rcu_irq_exit() is only called after this idle period to mark this cpu as > > > > rcu idle again. > > > > > > > > > Indeed. How about calling rcu_irq_exit() before native_safe_halt() and > > > rcu_irq_enter() right after? > > > > > We can't. If exception happens in the middle of rcu read section while > > preemption is disabled then calling rcu_irq_exit() before halt is > > incorrect. We can do that only if exception happen during idle and this > > should be rare enough for us to not care. > > If I understand correctly, maybe it doesn't cause problems. > > I guess you mean in other(not idle) task's rcu read critical section, we > get an async page fault? In this case the rcu_idle_exit() won't make > this cpu to enter rcu idle state. As the task environment is rcu non > idle. This is the case we use rcu_irq_enter()/rcu_irq_exit() in code > path where it might be in rcu idle or rcu non idle, and they don't > change the rcu non-idle state if used in rcu non-idle state. > > And it is also safe if it is rcu read critical section in idle, as it is > most probably wrapped in an outer rcu_irq_enter/exit(), so we have > > rcu idle > rcu_irq_enter() - exit rcu idle, to protect the read section > exception > rcu_irq_enter() in the page not present path > rcu_irq_exit() before halt > Yes, I was thinking about this scenario and you are right, since rcu_irq_enter() will be nested it should be safe to call rcu_irq_exit before halt. > so the halt is also safe here, as we have two rcu_irq_enter() and one > rcu_irq_exit(). > > I agree with Frederic that this is the safest and simplest way now, and > will try to update the code according to it. > > Thanks, Zhong > > > > >> > So I still want to remove it. And later if it shows that we really needs > > > >> > rcu somewhere in this code path, maybe we could use RCU_NONIDLE() to > > > >> > protect it. ( The suspicious RCU usage reported in commit > > > >> > c5e015d4949aa665 seems related to schedule(), which is not in the code > > > >> > path if we are in cpu idle eqs ) > > > >> > > > >> Yes but if rcu_irq_*() calls are fine to be called there, and I > > > >> believe they are because exception_enter() exits the user mode, we > > > >> should start to protect there right now instead of waiting for a > > > >> potential future warning of illegal RCU use. > > > > > > > > I agree, but I think by only protecting the necessary code avoids > > > > marking the entire waiting period as rcu non idle. > > > > > > If we use RCU_NONIDLE(), this assume we need to check all the code > > > there deeply for potential RCU uses and ensure it will never be > > > extended later to use RCU. We really don't scale enough for that. > > > There are too much subsystems involved there: waitqueue, spinlocks, > > > slab, per cpu, etc... > > > > > > So I strongly suggest we use rcu_irq_*() APIs, and relax them around > > > native_safe_halt() like I suggested above. This seems to me the safest > > > solution. > > > > -- > > Gleb. > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > > > -- Gleb. -- 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
