Messages in this thread Patch in this message | | | Date | Thu, 10 Jun 2021 17:48:13 -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 Fri, Jun 11, 2021 at 09:28:10AM +0900, Akira Yokosawa wrote: > On Thu, 10 Jun 2021 09:57:10 -0700, Paul E. McKenney wrote: > > 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. > > Hi Paul, > > See minor tweaks below to satisfy sphinx. > > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > 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; | > > This indent causes warnings from sphinx: > > Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst:135: WARNING: Unexpected indentation. > Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst:137: WARNING: Block quote ends without a blank line; unexpected unindent > > > +| 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 > > The code block in the answer can be fixed as follows: > > ++-----------------------------------------------------------------------+ > +| **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. | > ++-----------------------------------------------------------------------+ > > Hint: Use of "::" and indented code block.
Thank you!
As in with the following patch to be merged into Frederic's original, with attribution?
Thanx, Paul
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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 3cd5cb4d86e5..bc884ebf88bb 100644 --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst @@ -125,15 +125,15 @@ The ``smp_mb__after_unlock_lock()`` invocations prevent this +-----------------------------------------------------------------------+ | 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: | +| 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) | +| 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 |
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