On Wed, Jun 06, 2012 at 12:05:08PM -0400, Alan Stern wrote:
> 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);
> }
In this case, the ACCESS_ONCE() prevents the compiler from speculatively
executing the stuff following the "goto exit", which I freely admit is
insane even for compiler writers. But the documentation benefits still
stand.
> > 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.
I will reserve judgment until after I see what effect requiring all
globals to be SMP-safe has on code generation. ;-)
Thanx, Paul
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