Re: increased vmap_area_lock contentions on "n_tty: Move buffersinto n_tty_data"

[Peter Hurley]

On 09/26/2013 11:32 AM, Andi Kleen wrote:
> On Thu, Sep 26, 2013 at 07:52:23AM -0400, Peter Hurley wrote:
> > On 09/25/2013 11:20 PM, Andi Kleen wrote:
> > > Lin Ming <minggr@xxxxxxxxx> writes:
> > > > Would you like below patch?
> > >
> > > The loop body keeps rather complex state. It could easily
> > > get confused by parallel RCU changes.
> > >
> > > So if the list changes in parallel you may suddenly
> > > report very bogus values, as the va_start - prev_end
> > > computation may be bogus.
> > >
> > > Perhaps it's ok (may report bogus gaps), but it seems a bit risky.
> >
> > I don't understand how the computed gap would be bogus; there
> > _was_ a list state in which that particular gap existed. The fact
>
> It could change any time as you don't have an atomic view
> of vm_end / vm_start. It is valid to change the fields
> with the lock held.

va_start and va_end are constant for the lifetime of their vmap_area
(if it's accessible by traversing the vmap_area_list), so it is
not possible for an rcu-based list traversal to see different
values of these individual fields than the spin-locked version.

In addition, for the rcu-based traversal to have arrived at any given
vmap_area requires that the previous vmap_area was its adjacent
lower range at the instant in time when the list cursor was advanced;
again, this is no different than if the spin-locked version had
happened to begin at that same instant.

Regards,
Peter Hurley



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