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 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/ > -- 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
