On Tue, Oct 30, 2018 at 03:26:49PM -0700, Joel Fernandes wrote: > Hi Paul, > > On Sat, Oct 27, 2018 at 09:30:46PM -0700, Joel Fernandes (Google) wrote: > > As per this thread [1], it seems this smp_mb isn't needed anymore: > > "So the smp_mb() that I was trying to add doesn't need to be there." > > > > So let us remove this part from the memory ordering documentation. > > > > [1] https://lkml.org/lkml/2017/10/6/707 > > > > Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org> > > I was just checking about this patch. Do you feel it is correct to remove > this part from the docs? Are you satisified that a barrier isn't needed there > now? Or did I miss something?
Apologies, it got lost in the shuffle. I have now applied it with a bit of rework to the commit log, thank you! Thanx, Paul > thanks, > > - Joel > > > > --- > > .../Tree-RCU-Memory-Ordering.html | 32 +------------------ > > 1 file changed, 1 insertion(+), 31 deletions(-) > > > > diff --git > > a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html > > b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html > > index a346ce0116eb..0fb1511763d4 100644 > > --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html > > +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html > > @@ -77,7 +77,7 @@ The key point is that the lock-acquisition functions, > > including > > <tt>smp_mb__after_unlock_lock()</tt> immediately after successful > > acquisition of the lock. > > > > -<p>Therefore, for any given <tt>rcu_node</tt> struction, any access > > +<p>Therefore, for any given <tt>rcu_node</tt> structure, any access > > happening before one of the above lock-release functions will be seen > > by all CPUs as happening before any access happening after a later > > one of the above lock-acquisition functions. > > @@ -162,36 +162,6 @@ an <tt>atomic_add_return()</tt> of zero) to detect > > idle CPUs. > > <tr><td> </td></tr> > > </table> > > > > -<p>The approach must be extended to handle one final case, that > > -of waking a task blocked in <tt>synchronize_rcu()</tt>. > > -This task might be affinitied to a CPU that is not yet aware that > > -the grace period has ended, and thus might not yet be subject to > > -the grace period's memory ordering. > > -Therefore, there is an <tt>smp_mb()</tt> after the return from > > -<tt>wait_for_completion()</tt> in the <tt>synchronize_rcu()</tt> > > -code path. > > - > > -<table> > > -<tr><th> </th></tr> > > -<tr><th align="left">Quick Quiz:</th></tr> > > -<tr><td> > > - What? Where??? > > - I don't see any <tt>smp_mb()</tt> after the return from > > - <tt>wait_for_completion()</tt>!!! > > -</td></tr> > > -<tr><th align="left">Answer:</th></tr> > > -<tr><td bgcolor="#ffffff"><font color="ffffff"> > > - That would be because I spotted the need for that > > - <tt>smp_mb()</tt> during the creation of this documentation, > > - and it is therefore unlikely to hit mainline before v4.14. > > - Kudos to Lance Roy, Will Deacon, Peter Zijlstra, and > > - Jonathan Cameron for asking questions that sensitized me > > - to the rather elaborate sequence of events that demonstrate > > - the need for this memory barrier. > > -</font></td></tr> > > -<tr><td> </td></tr> > > -</table> > > - > > <p>Tree RCU's grace--period memory-ordering guarantees rely most > > heavily on the <tt>rcu_node</tt> structure's <tt>->lock</tt> > > field, so much so that it is necessary to abbreviate this pattern > > -- > > 2.19.1.568.g152ad8e336-goog > > >