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. > >> > 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-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
