Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

[Boqun Feng]

On Thu, Jul 27, 2017 at 07:36:58AM -0700, Paul E. McKenney wrote:
> > > 
> > > The reporting of the quiescent state will acquire the leaf rcu_node
> > > structure's lock, with an smp_mb__after_unlock_lock(), which will
> > > one way or another be a full memory barrier.  So the reorderings
> > > cannot happen.
> > > 
> > > Unless I am missing something subtle.  ;-)
> > > 
> > 
> > Well, smp_mb__after_unlock_lock() in ARM64 is a no-op, and ARM64's lock
> > doesn't provide a smp_mb().
> > 
> > So my point is more like: synchronize_sched() happens to be a
> > sys_membarrier() because of some implementation detail, and if some day
> > we come up with a much cheaper way to implement sched flavor
> > RCU(hopefully!), synchronize_sched() may be not good for the job. So at
> > least, we'd better document this somewhere?
> 
> Last I heard, ARM's unlock/lock acted as a full barrier.  Will?
> 
> Please see the synchronize_sched() comment header for the documentation
> you are asking for.  And the "Memory-Barrier Guarantees" section of
> Documentation/RCU/Design/Requirements/Requirements.html.
> 

All those barrier guarantees are subject to a RCU read-side critical
section with a synchonize_*(), IIRC, for example:

 * On systems with more than one CPU, when synchronize_sched() returns,
 * each CPU is guaranteed to have executed a full memory barrier since the
 * end of its last RCU-sched read-side critical section whose beginning
 * preceded the call to synchronize_sched().  In addition, each CPU having

, which is not the case for a quiesent state without a read-side
critical section(i.e. non-context-switch quiesent state for sched Flavor)

I've read those requirements and could not find one to explain why there
will be a full barrier emitted in an interrupted user-space program.

Regards,
Boqun

> 							Thanx, Paul
> 
> > Regards,
> > Boqun
> > 
> > > 						Thanx, Paul
> > > 
> > > > 					<return to user space>                        |    |
> > > > 					read Y; --------------------------------------+----+
> > > > 	store X;			                                              |
> > > > 					{read X}(reordered) <-------------------------+
> > > > 
> > > > I assume the timer interrupt handler, which interrupts a user space and
> > > > reports a quiesent state for sched flavor RCU, may not have a smp_mb()
> > > > in some code path.
> > > > 
> > > > I may miss something subtle, but it just not very obvious how
> > > > synchronize_sched() will guarantee a remote CPU running in userspace to
> > > > do a smp_mb() before it returns, this is at least not in RCU
> > > > requirements, right?
> > > > 
> > > > Regards,
> > > > Boqun
> > > 
> > > 
> 
>

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