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?
Mikulas
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