On Wed, Oct 24, 2012 at 04:57:35PM -0700, Paul E. McKenney wrote:
> On Wed, Oct 24, 2012 at 04:44:14PM -0400, Mikulas Patocka wrote:
> >
> >
> > On Wed, 24 Oct 2012, Paul E. McKenney wrote:
> >
> > > On Wed, Oct 24, 2012 at 04:22:17PM -0400, Mikulas Patocka wrote:
> > > >
> > > >
> > > > On Wed, 24 Oct 2012, Paul E. McKenney wrote:
> > > >
> > > > > On Tue, Oct 23, 2012 at 05:39:43PM -0400, Mikulas Patocka wrote:
> > > > > >
> > > > > >
> > > > > > 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".
> > > > >
> > > > > Perhaps I could, but it isn't needed, nor is it particularly helpful.
> > > > > Please see suggestions in preceding email.
> > > >
> > > > It is helpful, because if you add this requirement (that already holds
> > > > for
> > > > the current implementation), you can drop rcu_read_lock_sched() and
> > > > rcu_read_unlock_sched() from the following code that you submitted.
> > > >
> > > > static inline void percpu_up_read(struct percpu_rw_semaphore *p)
> > > > {
> > > > /*
> > > > * Decrement our count, but protected by RCU-sched so that
> > > > * the writer can force proper serialization.
> > > > */
> > > > rcu_read_lock_sched();
> > > > this_cpu_dec(*p->counters);
> > > > rcu_read_unlock_sched();
> > > > }
> > > >
> > > > > > The current implementation fulfills this requirement, you can just
> > > > > > add it
> > > > > > to the specification so that whoever changes the implementation
> > > > > > keeps it.
> > > > >
> > > > > I will consider doing that if and when someone shows me a situation
> > > > > where
> > > > > adding that requirement makes things simpler and/or faster. From
> > > > > what I
> > > > > can see, your example does not do so.
> > > > >
> > > > > Thanx, Paul
> > > >
> > > > If you do, the above code can be simplified to:
> > > > {
> > > > barrier();
> > > > this_cpu_dec(*p->counters);
> > > > }
> > >
> > > The readers are lightweight enough that you are worried about the overhead
> > > of rcu_read_lock_sched() and rcu_read_unlock_sched()? Really???
> > >
> > > Thanx, Paul
> >
> > There was no lock in previous kernels, so we should make it as simple as
> > possible. Disabling and reenabling preemption is probably not a big deal,
> > but if don't have to do it, why do it?
>
> Because I don't consider the barrier()-paired-with-synchronize_sched()
> to be a simplification.
In addition, please note that synchronize_srcu() used to guarantee a
memory barrier on all online non-idle CPUs, but that it no longer does
after Lai Jiangshan's recent rewrite. Given this change, I would have
to be quite foolish not to be very reluctant to make this guarantee for
other flavors of RCU, unless there was an extremely good reason for it.
Dropping a preempt_disable()/preempt_enable() pair doesn't even come
close to being a good enough reason.
> While we are discussing this, I have been assuming that readers must block
> from time to time. Is this the case?
And this really is a serious question. If the answer is "no", that
readers never block, a much simpler and faster approach is possible.
Thanx, Paul
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