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.
