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

Mathieu Desnoyers
2007-11-20T21:48:39Z

* Christoph Lameter (clameter@sgi.com) wrote:
> 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);
>

oops, I should remember not to review code before my first coffee in the
morning ;)

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68