On Fri, Jun 28, 2019 at 06:10:42PM +0900, Byungchul Park wrote: > On Fri, Jun 28, 2019 at 04:43:50PM +0900, Byungchul Park wrote: > > On Fri, Jun 28, 2019 at 04:31:38PM +0900, Byungchul Park wrote: > > > On Thu, Jun 27, 2019 at 01:36:12PM -0700, Paul E. McKenney wrote: > > > > On Thu, Jun 27, 2019 at 03:17:27PM -0500, Scott Wood wrote: > > > > > On Thu, 2019-06-27 at 11:41 -0700, Paul E. McKenney wrote: > > > > > > On Thu, Jun 27, 2019 at 02:16:38PM -0400, Joel Fernandes wrote: > > > > > > > > > > > > > > I think the fix should be to prevent the wake-up not based on whether we > > > > > > > are > > > > > > > in hard/soft-interrupt mode but that we are doing the rcu_read_unlock() > > > > > > > from > > > > > > > a scheduler path (if we can detect that) > > > > > > > > > > > > Or just don't do the wakeup at all, if it comes to that. I don't know > > > > > > of any way to determine whether rcu_read_unlock() is being called from > > > > > > the scheduler, but it has been some time since I asked Peter Zijlstra > > > > > > about that. > > > > > > > > > > > > Of course, unconditionally refusing to do the wakeup might not be happy > > > > > > thing for NO_HZ_FULL kernels that don't implement IRQ work. > > > > > > > > > > Couldn't smp_send_reschedule() be used instead? > > > > > > > > Good point. If current -rcu doesn't fix things for Sebastian's case, > > > > that would be well worth looking at. But there must be some reason > > > > why Peter Zijlstra didn't suggest it when he instead suggested using > > > > the IRQ work approach. > > > > > > > > Peter, thoughts? > > > > > > > +cc kernel-team@xxxxxxx > > (I'm sorry for more noise on the thread.) > > > > > Hello, > > > > > > Isn't the following scenario possible? > > > > > > The original code > > > ----------------- > > > rcu_read_lock(); > > > ... > > > /* Experdite */ > > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); > > > ... > > > __rcu_read_unlock(); > > > if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) > > > rcu_read_unlock_special(t); > > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); > > > rcu_preempt_deferred_qs_irqrestore(t, flags); > > > barrier(); /* ->rcu_read_unlock_special load before assign */ > > > t->rcu_read_lock_nesting = 0; > > > > > > The reordered code by machine > > > ----------------------------- > > > rcu_read_lock(); > > > ... > > > /* Experdite */ > > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); > > > ... > > > __rcu_read_unlock(); > > > if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) > > > rcu_read_unlock_special(t); > > > t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!! > > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); > > > rcu_preempt_deferred_qs_irqrestore(t, flags); > > > barrier(); /* ->rcu_read_unlock_special load before assign */ > > > > > > An interrupt happens > > > -------------------- > > > rcu_read_lock(); > > > ... > > > /* Experdite */ > > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); > > > ... > > > __rcu_read_unlock(); > > > if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) > > > rcu_read_unlock_special(t); > > > t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!! > > > <--- Handle an (any) irq > > > rcu_read_lock(); > > > /* This call should be skipped */ > > > rcu_read_unlock_special(t); > > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); > > > rcu_preempt_deferred_qs_irqrestore(t, flags); > > > barrier(); /* ->rcu_read_unlock_special load before assign */ > > I was confused it was a LOAD access. The example should be changed a bit. > > > > The original code > ----------------- > rcu_read_lock(); > ... > /* Experdite */ > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); > ... > __rcu_read_unlock(); > if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) > rcu_read_unlock_special(t); > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); > rcu_preempt_deferred_qs_irqrestore(t, flags); > barrier(); /* ->rcu_read_unlock_special load before assign */ > t->rcu_read_lock_nesting = 0; > > The reordered code by machine > ----------------------------- > rcu_read_lock(); > ... > /* Experdite */ > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); > ... > __rcu_read_unlock(); > if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) > rcu_read_unlock_special(t); > rcu_preempt_deferred_qs_irqrestore(t, flags); > barrier(); /* ->rcu_read_unlock_special load before assign */ > t->rcu_read_lock_nesting = 0; > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); > > An interrupt happens > -------------------- > rcu_read_lock(); > ... > /* Experdite */ > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); > ... > __rcu_read_unlock(); > if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) > rcu_read_unlock_special(t); > rcu_preempt_deferred_qs_irqrestore(t, flags); > barrier(); /* ->rcu_read_unlock_special load before assign */ > t->rcu_read_lock_nesting = 0; > <--- Handle an (any) irq > rcu_read_lock(); > /* This call should be skipped */ > rcu_read_unlock_special(t); > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); > > > > Now that I re-made the example, I'm afraid it'd be no problem because > anyway it'd be within a cpu so it can see inside of the store-buffer of > the cpu. > > I'm sorry. Please ignore my suggestion here. Even though the example is wrong but I think you can get confused with about what I was trying to tell. It was about (1) LOAD in advance and (2) reordering within a store buffer within a cpu, but not about reordering instructions - I wrote the example as if it's about the latter though. Sorry for noise again. Thanks, Byungchul
