On Wed, 6 Jun 2012, Paul E. McKenney wrote:
> On Wed, Jun 06, 2012 at 11:21:52AM -0400, Alan Stern wrote:
> > On Wed, 6 Jun 2012, Paul E. McKenney wrote:
> >
> > > No sane compiler would change it to a byte-at-a-time store, but the
> > > compiler would nevertheless be within its rights to do so. And a quick
> > > review of certain LKML threads could easily cause anyone to question gcc's
> > > sanity. Furthermore, the compiler is permitted to make transformations
> > > like the following, which it might well do to save a branch:
> > >
> > > if (b) a = 0;
> > > a = 1; if (b)
> > > else a = 1;
> > > a = 0;
> >
> > The compiler would be forbidden if the original code were
> >
> > if (b)
> > ACCESS_ONCE(a) = 1;
> > else
> > ACCESS_ONCE(a) = 0;
> >
> > But if I remember correctly, the code snippet we were talking was more
> > like:
> >
> > if (ACCESS_ONCE(b))
> > a = 1;
> >
> > Isn't this use of ACCESS_ONCE unnecessary?
>
> That would depend on what else is nearby.
Here's the relevant part of the original patch:
@@ -467,6 +473,12 @@ EXPORT_SYMBOL_GPL(driver_attach);
static void __device_release_driver(struct device *dev)
{
struct device_driver *drv;
+ int idx;
+
+ idx = srcu_read_lock(&driver_srcu);
+
+ if (ACCESS_ONCE(device_shutdown_started))
+ goto exit;
drv = dev->driver;
if (drv) {
@@ -494,6 +506,8 @@ static void __device_release_driver(struct device *dev)
dev);
}
+exit:
+ srcu_read_unlock(&driver_srcu, idx);
}
> There are some limitations because volatile accesses are not allowed to
> move past "sequence points", but it would be possible to come up with
> similar examples. This sort of thing is why C1x has a memory model and
> why it allows variables to be designated as needing to be SMP-safe.
Almost certainly the kernel won't use this facility. Or else it will
just require that _all_ global variables be SMP-safe.
Alan Stern
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