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>

kernel/rcu/srcutree.c

@@ -705,9 +705,15 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 	int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
 	unsigned long sgsne;
 	struct srcu_node *snp;
-	struct srcu_node *snp_leaf = smp_load_acquire(&sdp->mynode);
+	struct srcu_node *snp_leaf;
 	unsigned long snp_seq;
 
+	/* Ensure that snp node tree is fully initialized before traversing it */
+	if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
+		snp_leaf = NULL;
+	else
+		snp_leaf = sdp->mynode;
+
 	if (snp_leaf)
 		/* Each pass through the loop does one level of the srcu_node tree. */
 		for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
@@ -889,10 +895,13 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 	bool needgp = false;
 	unsigned long s;
 	struct srcu_data *sdp;
+	struct srcu_node *sdp_mynode;
+	int ss_state;
 
 	check_init_srcu_struct(ssp);
 	idx = srcu_read_lock(ssp);
-	if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_CALL)
+	ss_state = smp_load_acquire(&ssp->srcu_size_state);
+	if (ss_state < SRCU_SIZE_WAIT_CALL)
 		sdp = per_cpu_ptr(ssp->sda, 0);
 	else
 		sdp = raw_cpu_ptr(ssp->sda);
@@ -912,10 +921,17 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 		needexp = true;
 	}
 	spin_unlock_irqrestore_rcu_node(sdp, flags);
+
+	/* Ensure that snp node tree is fully initialized before traversing it */
+	if (ss_state < SRCU_SIZE_WAIT_BARRIER)
+		sdp_mynode = NULL;
+	else
+		sdp_mynode = sdp->mynode;
+
 	if (needgp)
 		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
 	else if (needexp)
-		srcu_funnel_exp_start(ssp, smp_load_acquire(&sdp->mynode), s);
+		srcu_funnel_exp_start(ssp, sdp_mynode, s);
 	srcu_read_unlock(ssp, idx);
 	return s;
 }