Re: [rfc 04/45] cpu alloc: Use in SLUB

Christoph Lameter
2007-11-20T21:37:10Z

On Tue, 20 Nov 2007, Mathieu Desnoyers wrote:

> > > > - __slab_free(s, page, x, addr, c->offset);
> > > > + if (unlikely(page != __CPU_READ(c->page) ||
> > > > + __CPU_READ(c->node) < 0)) {
> > > > + __slab_free(s, page, x, addr, __CPU_READ(c->offset));
> > >
> > > And same question as above : what happens if we fail after executing the
> > > __slab_free.. is it valid to do it twice ?
> >
> > __slab_free is always successful and will never cause a repeat of the
> > loop.
>
> Then what happens if we are migrated between the end of __slab_free and
> the CPU_CMPXCHG ? The cmpxchg would fail, and the __slab_free will be
> done twice ?

Look at the control flow. The __slab_free can never be done twice.

{
freelist = __CPU_READ(c->freelist);
barrier();
if (unlikely(page != __CPU_READ(c->page) ||
__CPU_READ(c->node) < 0)) {
__slab_free(s, page, x, addr);
break;
^^^^^^^ no redo of the loop possible.

}
set_freepointer(s, object, freelist);
} while (CPU_CMPXCHG(c->freelist, freelist, object) != freelist);