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

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Date:	Tue, 20 Nov 2007 13:36:58 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	ak@...e.de, akpm@...ux-foundation.org, travis@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc 04/45] cpu alloc: Use in SLUB

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);

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