On Wed, Aug 3, 2011 at 7:37 AM, Mulyadi Santosa <mulyadi.santosa@xxxxxxxxx> wrote: > Dear Julie.... > > On Tue, Aug 2, 2011 at 04:28, Julie Sullivan <kernelmail.jms@xxxxxxxxx> wrote: >> 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 _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
