linux-kernel - [PATCH rcu 06/19] srcu: Add size-state transitioning code

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Date:   Fri,  4 Feb 2022 15:38:49 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 06/19] srcu: Add size-state transitioning code

This is just dead code at the moment, but it serves to prevent
spurious compiler warnings about init_srcu_struct_nodes() being unused.
This function will once again be used once the state-transition code
is activated.

Because srcu_barrier() must be aware of transition before call_srcu(), the
state machine waits for an SRCU grace period before callbacks are queued
to the non-CPU-0 queues.  This requres that portions of srcu_barrier()
be enclosed in an SRCU read-side critical section.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/srcutree.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 2bbe8a5d9ae86..fce041b648ee3 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -202,7 +202,6 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	if (!ssp->sda)
 		return -ENOMEM;
 	init_srcu_struct_data(ssp);
-	WARN_ON_ONCE(!init_srcu_struct_nodes(ssp));
 	ssp->srcu_gp_seq_needed_exp = 0;
 	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
 	smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
@@ -555,6 +554,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
 	unsigned long mask;
 	struct srcu_data *sdp;
 	struct srcu_node *snp;
+	int ss_state;
 
 	/* Prevent more than one additional grace period. */
 	mutex_lock(&ssp->srcu_cb_mutex);
@@ -622,6 +622,15 @@ static void srcu_gp_end(struct srcu_struct *ssp)
 	} else {
 		spin_unlock_irq_rcu_node(ssp);
 	}
+
+	/* Transition to big if needed. */
+	ss_state = smp_load_acquire(&ssp->srcu_size_state);
+	if (ss_state && ss_state != SRCU_SIZE_BIG) {
+		if (ss_state == SRCU_SIZE_ALLOC)
+			init_srcu_struct_nodes(ssp);
+		else
+			smp_store_release(&ssp->srcu_size_state, ss_state + 1);
+	}
 }
 
 /*
@@ -1172,6 +1181,7 @@ static void srcu_barrier_one_cpu(struct srcu_struct *ssp, struct srcu_data *sdp)
 void srcu_barrier(struct srcu_struct *ssp)
 {
 	int cpu;
+	int idx;
 	unsigned long s = rcu_seq_snap(&ssp->srcu_barrier_seq);
 
 	check_init_srcu_struct(ssp);
@@ -1187,11 +1197,13 @@ void srcu_barrier(struct srcu_struct *ssp)
 	/* Initial count prevents reaching zero until all CBs are posted. */
 	atomic_set(&ssp->srcu_barrier_cpu_cnt, 1);
 
+	idx = srcu_read_lock(ssp);
 	if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
 		srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, 0));
 	else
 		for_each_possible_cpu(cpu)
 			srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, cpu));
+	srcu_read_unlock(ssp, idx);
 
 	/* Remove the initial count, at which point reaching zero can happen. */
 	if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
-- 
2.31.1.189.g2e36527f23