Messages in this thread Patch in this message | | | Date | Thu, 10 Jun 2021 09:57:10 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock_lock() |
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On Thu, Jun 10, 2021 at 05:50:29PM +0200, Frederic Weisbecker wrote: > Add some missing critical pieces of explanation to understand the need > for full memory barriers throughout the whole grace period state machine, > thanks to Paul's explanations. > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > Cc: Neeraj Upadhyay <neeraju@codeaurora.org> > Cc: Joel Fernandes <joel@joelfernandes.org> > Cc: Uladzislau Rezki <urezki@gmail.com> > Cc: Boqun Feng <boqun.feng@gmail.com>
Nice!!! And not bad wording either, though I still could not resist the urge to wordsmith further. Plus I combined your two examples, in order to provide a trivial example use of the polling interfaces, if nothing else.
Please let me know if I messed anything up.
Thanx, Paul
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commit f21b8fbdf9a59553da825265e92cedb639b4ba3c Author: Frederic Weisbecker <frederic@kernel.org> Date: Thu Jun 10 17:50:29 2021 +0200
rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock_lock() Add some missing critical pieces of explanation to understand the need for full memory barriers throughout the whole grace period state machine, thanks to Paul's explanations. Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Cc: Neeraj Upadhyay <neeraju@codeaurora.org> Cc: Joel Fernandes <joel@joelfernandes.org> Cc: Uladzislau Rezki <urezki@gmail.com> Cc: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst index 11cdab037bff..3cd5cb4d86e5 100644 --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst @@ -112,6 +112,35 @@ on PowerPC. The ``smp_mb__after_unlock_lock()`` invocations prevent this ``WARN_ON()`` from triggering. ++-----------------------------------------------------------------------+ +| **Quick Quiz**: | ++-----------------------------------------------------------------------+ +| But the whole chain of rcu_node-structure locking guarantees that | +| readers see all pre-grace-period accesses from the updater and | +| also guarantees that the updater to see all post-grace-period | +| accesses from the readers. 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(). For example: | +| | +| 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 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. | ++-----------------------------------------------------------------------+ + 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
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