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

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Message-ID: <20071120214319.GA13825@Krystal>
Date:	Tue, 20 Nov 2007 16:43:20 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
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

* Christoph Lameter (clameter@....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
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