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Subject: Re: [thiscpuops upgrade 08/10] percpu: generic this_cpu_cmpxchg()
 and this_cpu_cmpxchg_double support
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>, akpm@linux-foundation.org,
        Pekka Enberg <penberg@cs.helsinki.fi>, linux-kernel@vger.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
In-Reply-To: <4CEFE583.8070103@kernel.org>
References: <20101123235139.908255844@linux.com>
	 <20101123235200.582485207@linux.com>  <4CEFE583.8070103@kernel.org>
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Date: Fri, 26 Nov 2010 17:56:09 +0100
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Le vendredi 26 novembre 2010 à 17:51 +0100, Tejun Heo a écrit :
> On 11/24/2010 12:51 AM, Christoph Lameter wrote:
> > +/*
> > + * cmpxchg_double replaces two adjacent scalars at once. The first parameter
> > + * passed is a percpu pointer, not a scalar like the other this_cpu
> > + * operations. This is so because the function operates on two scalars
> > + * (must be of same size). A truth value is returned to indicate success or
> > + * failure (since a double register result is difficult to handle).
> > + * There is very limited hardware support for these operations. So only certain
> > + * sizes may work.
> > + */
> > +#define __this_cpu_generic_cmpxchg_double(pcp, oval1, oval2, nval1, nval2)	\
> 
> Ah... this is scary.  If it proves to be useful enough, sure, but
> otherwise I'd like to avoid it.
> 

This is mandatory for several lockless algos, dont be afraid, its a
single instruction ;)