Le mardi 17 août 2010 à 16:46 +0800, Changli Gao a écrit :
> On Tue, Aug 17, 2010 at 4:30 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> >
> > Three variables ?
> >
> > static atomic_t rnd __read_mostly;
> >
> > if (unlikely(!atomic_read(&rnd))) {
> > unsigned int val;
> >
> > get_random_bytes(&val, sizeof(val));
> > if (!val)
> > val = 1;
> > atomic_cmpxchg(&rnd, 0, val);
> > }
> >
>
>
> Good idea. However, atomic_t is a volatile variable, and it prevent
> ILP. I think maybe it hurts the likely case. cmpxchg() is an atomic
> operations, so is the bellow code better?
>
I am not sure what you mean by ILP.
On x86, reading twice same memory location is faster than
reading it and store in temp variable (on stack)
reading from stack
> static unsigned long rnd __read_mostly;
>
> if (unlikely(!rnd)) {
> unsigned long val;
>
> get_random_bytes(&val, sizeof(val));
> if (!val)
> val = 1;
> cmpxchg(&rnd, 0, val);
> }
>
> Thanks.
>
I am not sure we must use a long (we really need 4 bytes only), and last
time I tried to use cmpxchg(), I was being told it was not available on
all arches.
But seeing it used in kernel/pid.c, maybe its not true anymore (that is,
__HAVE_ARCH_CMPXCHG is always defined to 1)
Since its a recent change (in kernel/pid.c), I would wait a bit and see
if an arch maintainer complains ;)
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