Hi list, I'm looking at RCU at the moment and (on reading a recent article by Paul McKenney on LWN) I've spotted something I'm a bit confused by in this code, a function which is defined in kernel/rcutree_plugin.h: void __rcu_read_unlock(void) { struct task_struct *t = current; barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */ --t->rcu_read_lock_nesting; barrier(); /* decrement before load of ->rcu_read_unlock_special */ if (t->rcu_read_lock_nesting == 0 && unlikely(ACCESS_ONCE(t->rcu_read_unlock_special))) rcu_read_unlock_special(t); #ifdef CONFIG_PROVE_LOCKING WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0); #endif /* #ifdef CONFIG_PROVE_LOCKING */ } 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? Because both must be evaluated, with left-to-right associativity, and then both must be true for the branch to be taken, right? If it's necessary for branch prediction/optimization purposes shouldn't it be applied to both or the first one? Or might the operands also be re-ordered by the compiler or processor so that the second is evaluated before the first? What am I misunderstanding about this attribute? Cheers Julie