[PATCHv3] srcu: Eliminate the case that snp_seq bigger than snap in srcu_funnel_gp_start()

[Pingfan Liu <kernelfans@gmail.com>]

Since the srcu read lock is still held during srcu_funnel_gp_start(),
the seq snap should be the largest number for the slot
srcu_have_cbs[idx].

Test info
Running the following test without any failure
  tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 10h --configs 18*SRCU-P
And
  cd res && find . -name  console.log -exec grep WARNING {} \;
prints nothing

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: rcu@vger.kernel.org
---
 kernel/rcu/srcutree.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 7e450bb4ddcf..687f243dd2e2 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -905,14 +905,12 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 		for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
 			spin_lock_irqsave_rcu_node(snp, flags);
 			snp_seq = snp->srcu_have_cbs[idx];
-			if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
+			/* s should be the biggest in the current slot. */
+			WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq));
+			if (!srcu_invl_snp_seq(snp_seq) && (snp_seq == s)) {
 				if (snp == snp_leaf && snp_seq == s)
 					snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
 				spin_unlock_irqrestore_rcu_node(snp, flags);
-				if (snp == snp_leaf && snp_seq != s) {
-					srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0);
-					return;
-				}
 				if (!do_norm)
 					srcu_funnel_exp_start(ssp, snp, s);
 				return;
-- 
2.31.1