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

[Boqun Feng <boqun.feng@gmail.com>]

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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>    |    |
					<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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