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
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