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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jul 27, 2017 at 10:47:03PM +0800, Boqun Feng wrote:
> 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.

What you are forgetting is that for synchronize_sched(), any region of
code with preemption disabled is an RCU-sched read-side critical section.

							Thanx, Paul

> 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
> > > > 
> > > > 
> > 
> >