Messages in this thread | | | Subject | Re: [thiscpuops upgrade 08/10] percpu: generic this_cpu_cmpxchg() and this_cpu_cmpxchg_double support | From | Eric Dumazet <> | Date | Fri, 26 Nov 2010 18:01:27 +0100 |
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Le vendredi 26 novembre 2010 à 17:58 +0100, Tejun Heo a écrit : > On 11/26/2010 05:56 PM, Eric Dumazet wrote: > > 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 ;) > > Thanks for holding my hands. :-) > > What I'm afraid of is generic code switching to use it in very hot > paths when a lot of archs can't actually do it leading to performance > regressions. Is this something which is available on most archs? >
I would say cmpxchg() has the same problem, some arches dont have a native instruction, yet we use it in some paths.
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