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SubjectRe: [PATCH] rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock_lock()
On Sat, Jun 12, 2021 at 12:45:17AM +0200, Frederic Weisbecker wrote:
> On Fri, Jun 11, 2021 at 10:25:14AM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 11, 2021 at 12:34:32PM +0200, Frederic Weisbecker wrote:
> > Glad to help, and I will reach out to you should someone make the mistake
> > of insisting that I write something in French. ;-)
>
> If that can help, we still have frenglish for neutral territories such as airports.
> Not easy to master though...

That does sound dangerous! ;-)

> > > > ++-----------------------------------------------------------------------+
> > > > +
> > > > This approach must be extended to include idle CPUs, which need
> > > > RCU's grace-period memory ordering guarantee to extend to any
> > > > RCU read-side critical sections preceding and following the current
> >
> > How about like this?
> >
> > +-----------------------------------------------------------------------+
> > | **Quick Quiz**: |
> > +-----------------------------------------------------------------------+
> > | But the chain of rcu_node-structure lock acquisitions guarantees |
> > | that new readers will see all of the updater's pre-grace-period |
> > | accesses and also guarantees that the updater's post-grace-period |
> > | accesses will see all of the old reader's accesses. So why do we |
> > | need all of those calls to smp_mb__after_unlock_lock()? |
> > +-----------------------------------------------------------------------+
> > | **Answer**: |
> > +-----------------------------------------------------------------------+
> > | Because we must provide ordering for RCU's polling grace-period |
> > | primitives, for example, get_state_synchronize_rcu() and |
> > | poll_state_synchronize_rcu(). Consider this code:: |
> > | |
> > | CPU 0 CPU 1 |
> > | ---- ---- |
> > | WRITE_ONCE(X, 1) WRITE_ONCE(Y, 1) |
> > | g = get_state_synchronize_rcu() smp_mb() |
> > | while (!poll_state_synchronize_rcu(g)) r1 = READ_ONCE(X) |
> > | continue; |
> > | r0 = READ_ONCE(Y) |
> > | |
> > | RCU guarantees that the outcome r0 == 0 && r1 == 0 will not |
> > | happen, even if CPU 1 is in an RCU extended quiescent state |
> > | (idle or offline) and thus won't interact directly with the RCU |
> > | core processing at all. |
> > +-----------------------------------------------------------------------+
>
> Very good, thanks a lot :o)

And thank you!

Thanx, Paul

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