srcu: Ensure snp nodes tree is fully initialized before traversal

srcu: Ensure snp nodes tree is fully initialized before traversal

For configurations where snp node tree is not initialized at
init time (added in subsequent commits), srcu_funnel_gp_start()
and srcu_funnel_exp_start() can potential traverse and observe
the snp nodes' transient (uninitialized) states. This can potentially
happen, when init_srcu_struct_nodes() initialization of sdp->mynode
races with srcu_funnel_gp_start() and srcu_funnel_exp_start()

Consider the case below where srcu_funnel_gp_start() observes
sdp->mynode to be not NULL and uses an uninitialized sdp->grpmask

          P1                                  P2

init_srcu_struct_nodes()           void srcu_funnel_gp_start(...)
{
for_each_possible_cpu(cpu) {
  ...
  sdp->mynode = &snp_first[...];
  for (snp = sdp->mynode;...)       struct srcu_node *snp_leaf =
                                       smp_load_acquire(&sdp->mynode)
    ...                             if (snp_leaf) {
                                      for (snp = snp_leaf; ...)
                                        ...
					if (snp == snp_leaf)
                                         snp->srcu_data_have_cbs[idx] |=
                                           sdp->grpmask;
    sdp->grpmask =
      1 << (cpu - sdp->mynode->grplo);
  }
}

Similarly, init_srcu_struct_nodes() and srcu_funnel_exp_start() can
race, where srcu_funnel_exp_start() could observe state of snp lock
before spin_lock_init().

          P1                                      P2

init_srcu_struct_nodes()               void srcu_funnel_exp_start(...)
{
  srcu_for_each_node_breadth_first(ssp, snp) {      for (; ...) {
                                                      spin_lock_...(snp, )
	spin_lock_init(&ACCESS_PRIVATE(snp, lock));
    ...
  }
  for_each_possible_cpu(cpu) {
    ...
    sdp->mynode = &snp_first[...];

To avoid these issues, ensure that snp node tree initialization is
complete i.e. after SRCU_SIZE_WAIT_BARRIER srcu_size_state is reached,
before traversing the tree. Given that srcu_funnel_gp_start() and
srcu_funnel_exp_start() are called within SRCU read side critical
sections, this check is safe, in the sense that all callbacks are
enqueued on CPU0 srcu_cblist until SRCU_SIZE_WAIT_CALL is entered,
and these read side critical sections (containing srcu_funnel_gp_start()
and srcu_funnel_exp_start()) need to complete, before SRCU_SIZE_WAIT_CALL
is reached.

Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>