linux-kernel - Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

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Date:	Tue, 23 Oct 2012 17:39:43 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers



On Tue, 23 Oct 2012, Paul E. McKenney wrote:

> On Tue, Oct 23, 2012 at 01:29:02PM -0700, Paul E. McKenney wrote:
> > On Tue, Oct 23, 2012 at 08:41:23PM +0200, Oleg Nesterov wrote:
> > > On 10/23, Paul E. McKenney wrote:
> > > >
> > > >  * Note that this guarantee implies a further memory-ordering guarantee.
> > > >  * 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 read-side critical section
> > >          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > 
> > > Ah wait... I misread this comment.
> > 
> > And I miswrote it.  It should say "since the end of its last RCU-sched
> > read-side critical section."  So, for example, RCU-sched need not force
> > a CPU that is idle, offline, or (eventually) executing in user mode to
> > execute a memory barrier.  Fixed this.

Or you can write "each CPU that is executing a kernel code is guaranteed 
to have executed a full memory barrier".

It would be consistent with the current implementation and it would make 
it possible to use

barrier()-synchronize_sched() as biased memory barriers.

---

In percpu-rwlocks, CPU 1 executes

...make some writes in the critical section...
barrier();
this_cpu_dec(*p->counters);

and the CPU 2 executes

while (__percpu_count(p->counters))
	msleep(1);
synchronize_sched();

So, when CPU 2 finishes synchronize_sched(), we must make sure that
all writes done by CPU 1 are visible to CPU 2.

The current implementation fulfills this requirement, you can just add it 
to the specification so that whoever changes the implementation keeps it.

Mikulas

> And I should hasten to add that for synchronize_sched(), disabling
> preemption (including disabling irqs, further including NMI handlers)
> acts as an RCU-sched read-side critical section.  (This is in the
> comment header for synchronize_sched() up above my addition to it.)
> 	
> 							Thanx, Paul
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