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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
