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

[Boqun Feng]

On Thu, Jul 27, 2017 at 07:16:33AM -0700, Paul E. McKenney wrote:
> On Thu, Jul 27, 2017 at 09:55:51PM +0800, Boqun Feng wrote:
> > Hi Paul,
> > 
> > I have a side question out of curiosity:
> > 
> > How does synchronize_sched() work properly for sys_membarrier()?
> > 
> > sys_membarrier() requires every other CPU does a smp_mb() before it
> > returns, and I know synchronize_sched() will wait until all CPUs running
> > a kernel thread do a context-switch, which has a smp_mb(). However, I
> > believe sched flavor RCU treat CPU running a user thread as a quiesent
> > state, so synchronize_sched() could return without that CPU does a
> > context switch. 
> > 
> > So why could we use synchronize_sched() for sys_membarrier()?
> > 
> > In particular, could the following happens?
> > 
> > 	CPU 0:				CPU 1:
> > 	=========================	==========================
> > 	<in user space>			<in user space>
> > 					{read Y}(reordered) <------------------------------+
> > 	store Y;			                                                   |
> > 					read X; --------------------------------------+    |
> > 	sys_membarrier():		<timer interrupt>                             |    |
> > 	  synchronize_sched();		  update_process_times(user): //user == true  |    |
> > 	  				    rcu_check_callbacks(usr):                 |    |
> > 					      if (user || ..) {                       |    |
> > 					        rcu_sched_qs()                        |    |
> > 						...                                   |    |
> > 						<report quesient state in softirq>    |    |
> 
> 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?

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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