linux-kernel - [PATCH 1/4] rcu: Ignore rdp.cpu_no_qs.b.exp on premptible RCU's rcu

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Message-Id: <20210916121048.36623-2-frederic@kernel.org>
Date:   Thu, 16 Sep 2021 14:10:45 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>, rcu@...r.kernel.org,
        urezki@...il.com, boqun.feng@...il.com,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        joel@...lfernandes.org
Subject: [PATCH 1/4] rcu: Ignore rdp.cpu_no_qs.b.exp on premptible RCU's rcu_qs()

The preemptible RCU flavour doesn't rely at all on
struct rcu_data::cpu_no_qs::b::exp and use its own
struct rcu_data::exp_deferred_qs field to record the need for an
expedited quiescent state.

In fact rdp.cpu_no_qs.b.exp should never be set in preemptible RCU.

Make that fact clear and disambiguate the expectations on the
preemptible RCU's rcu_qs() implementation. In this flavour, common
quiescent states aren't expected to deal with expedited grace periods
because those have their own way to report expedited quiescent states:
rcu_read_unlock_special, context switches, etc...

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/rcu/tree_plugin.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1a6fdb03d0a5..4c9aeabec242 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -277,12 +277,16 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
  * current task, there might be any number of other tasks blocked while
  * in an RCU read-side critical section.
  *
+ * Unlike non-preemptible-RCU, quiescent state reports for expedited
+ * grace periods are handled seperately via deferred quiescent states
+ * and context switch events.
+ *
  * Callers to this function must disable preemption.
  */
 static void rcu_qs(void)
 {
 	RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
-	if (__this_cpu_read(rcu_data.cpu_no_qs.s)) {
+	if (__this_cpu_read(rcu_data.cpu_no_qs.b.norm)) {
 		trace_rcu_grace_period(TPS("rcu_preempt"),
 				       __this_cpu_read(rcu_data.gp_seq),
 				       TPS("cpuqs"));
-- 
2.25.1