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:
Here is your nice elegant little algorithm equipped to survive within
the Linux kernel:
Any questions? ;-)