Am Donnerstag, 12. April 2007 08:27 schrieb Greg KH:
> On Mon, Apr 02, 2007 at 04:33:54PM +0200, Eric Dumazet wrote:
> > On Mon, 2 Apr 2007 14:47:59 +0200
> > Oliver Neukum <[EMAIL PROTECTED]> wrote:
> >
> > > Hi,
> > >
> > > some atomic operations are only atomic, not ordered. Thus a CPU is allowed
> > > to reorder memory references to an object to before the reference is
> > > obtained. This fixes it.
> > >
> > > Regards
> > > Oliver
> > > Signed-off-by: Oliver Neukum <[EMAIL PROTECTED]>
> > > ------
> > >
> > > --- a/lib/kref.c 2007-04-02 14:40:40.000000000 +0200
> > > +++ b/lib/kref.c 2007-04-02 14:40:50.000000000 +0200
> > > @@ -21,6 +21,7 @@
> > > void kref_init(struct kref *kref)
> > > {
> > > atomic_set(&kref->refcount,1);
> > > + smp_mb();
> > > }
> >
> > I dont understand why smp_mb() is needed here, and not in
> > spinlock_init() for example.
>
> I think, after reading the Documentation/memory-barriers.txt and
> Documentation/atomic_ops.txt documentation, that spin_lock_init() also
> needs this kind of memory barrier.
spin_lock_init() is not an atomic operation.
In principle, the issue exists. However, the whole issue is a bit of a grey
area. You might take the viewpoint that upping the refcount needs to be
under lock, which needs to take care of ordering issues in case of krefs.
A new spinlock has the same issue. You need to be careful making them
accessible to other CPUs.
If you take code like:
static int producer()
{
...
data = kmalloc(...);
spin_lock_init(&data->lock);
data->value = some_value;
data->next = global_pointer;
global_pointer = data;
...
}
You have an ordering bug anyway, which you can't fix in spin_lock_init().
Regards
Oliver
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