On Fri, May 04, 2018 at 12:57:19PM -0700, Joel Fernandes wrote: > On Fri, May 4, 2018 at 11:49 AM, Paul E. McKenney > <paul...@linux.vnet.ibm.com> wrote: > > On Fri, May 04, 2018 at 06:34:32PM +0000, Joel Fernandes wrote: > >> On Fri, May 4, 2018 at 10:42 AM Paul E. McKenney > >> <paul...@linux.vnet.ibm.com> > >> wrote: > >> [...] > >> > > > > But preemptible RCU *does not* use context-switch as a quiescent > >> state. > >> > > > It doesn't? > >> > > > >> > > I thought that's what preemptible rcu is about. You can get preempted > >> but > >> > > you shouldn't block in a read-section. Is that not true? > >> > >> > Almost. All context switches in an RCU-preempt read-side critical > >> > section > >> > must be subject to priority boosting. Preemption is one example, because > >> > boosting the priority of the preempted task will make it runnable. > >> > The priority-inheritance -rt "spinlock" is another example, because > >> > boosting the priority of the task holding the lock will eventually make > >> > runnable the task acquiring the lock within the RCU-preempt read-side > >> > critical section. > >> > >> Yes I understand priority boosting is needed with preemptible RCU so that > >> read-sections are making forward progress. I meant (and correct me if I'm > >> wrong) that, as long as a task doesn't sleep in a preemptible RCU > >> read-section (rcu-preempt flavor), then bad things wont happen and RCU will > >> work correctly. > > > > The exception is -rt "spinlock" acquisition. If the "spinlock" is held, > > the task acquiring it will block, which is legal within an RCU-preempt > > read-side critical section. > > > > This exception is why I define bad things in terms of lack of > > susceptibility to priority boosting instead of sleeping. > > Oh, that's a tricky situation. Thanks for letting me know. I guess my > view was too idealistic. Makes sense now.
Well, let me put it this way... Here is your nice elegant little algorithm: https://upload.wikimedia.org/wikipedia/commons/6/6e/Golde33443.jpg Here is your nice elegant little algorithm equipped to survive within the Linux kernel: https://totatema.files.wordpress.com/2012/06/feeling_grizzly-1600x1200.jpg Any questions? ;-) Thanx, Paul
