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.
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
> > But this patch needs more? Or I misunderstood. There is no RCU unlock
> > in percpu_up_read().
> >
> > IOW. Suppose the code does
> >
> > percpu_down_read();
> > x = PROTECTED_BY_THIS_RW_SEM;
> > percpu_up_read();
> >
> > Withoit mb() the load above can be reordered with this_cpu_dec() in
> > percpu_up_read().
> >
> > However, we do not care if we can guarantee that the next
> > percpu_down_write() can not return (iow, the next "write" section can
> > not start) until this load is complete.
> >
> > And I _think_ that another synchronize_sched() in percpu_down_write()
> > added by this patch should work.
> >
> > But, "since the end of its last RCU read-side critical section"
> > does not look enough.
> >
> > Or I misundersood you/Mikulas/both ?
>
> I clearly need to look more carefully at Mikulas's code...
>
> 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/
