On Wed, Aug 3, 2011 at 7:37 AM, Mulyadi Santosa
[off-list ref] wrote:

Dear Julie....

On Tue, Aug 2, 2011 at 04:28, Julie Sullivan [off-list ref] wrote:

quoted

Specifically, the conditional

?if (t->rcu_read_lock_nesting == 0 &&
? ? ? ? ? ?unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
? ? ? ? ? ? ? ?rcu_read_unlock_special(t);

I've googled for 'likely/unlikely' and i've seen

?if (unlikely(...

before in kernel code, and I might understand if the logical operator
here were || not &&, but why is the 'unlikely' attribute only applied
to the second operand here?

i guess that's because first condition "t->rcu_read_lock_nesting == 0"
has about 50%-50% chance.... so putting likely() or unlikely() has no
use here...

i read somewhere (or maybe someone's opinion) that you only use
likely() or unlikely() if you are about above 90% (or maybe...if
possible 99%) sure that condition will be true or false....otherwise,
avoid likely()/unlikely() and let the compiler+processor do the usual
ordering/branch prediction.

just my 2 cents...

--
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

Thanks Mulyadi, maybe someday when I get round to performance testing
I'll try this out for myself...


Cheers
Julie