256 If you are going to be fetching multiple fields from the
257 RCU-protected structure, using the local variable is of
258 course preferred. Repeated rcu_dereference() calls look
259 ugly and incur unnecessary overhead on Alpha CPUs.”
From lines 256 to 259 I conclude that reader()’s code is considered ugly and wasteful,
but a will always equal b.
But looking at how rcu_dereference() and rcu_assign_pointer() are implemented, I’m having a
hard time seeing how reader() would always see a and b equal.
This is the implementation of rcu_dereference(). It is a little old, but useful as well. #define rcu_dereference(p) ({ \ typeof(p) _________p1 = ACCESS_ONCE(p); \ smp_read_barrier_depends(); \ (_________p1); \ }) It uses memory barrier to guarantee the order of code execution. rcu_read_lock() actually disables preemption, so writer has no chance to modify critical section in the rcu_read_lock()/rcu_read_unlock() pair.