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Date:   Fri, 14 Feb 2020 15:56:06 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 29/30] rcu: Mark rcu_state.gp_seq to detect concurrent writes

From: "Paul E. McKenney" <paulmck@...nel.org>

The rcu_state structure's gp_seq field is only to be modified by the RCU
grace-period kthread, which is single-threaded.  This commit therefore
enlists KCSAN's help in enforcing this restriction.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3a60a160..d15041f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1209,7 +1209,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
 		trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
 		goto unlock_out;
 	}
-	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rcu_state.gp_seq), TPS("newreq"));
+	trace_rcu_grace_period(rcu_state.name, data_race(rcu_state.gp_seq), TPS("newreq"));
 	ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
 	/* Push furthest requested GP to leaf node and rcu_data structure. */
@@ -1494,6 +1494,7 @@ static bool rcu_gp_init(void)
 	record_gp_stall_check_time();
 	/* Record GP times before starting GP, hence rcu_seq_start(). */
 	rcu_seq_start(&rcu_state.gp_seq);
+	ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
 	trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
 	raw_spin_unlock_irq_rcu_node(rnp);
 
@@ -1656,8 +1657,7 @@ static void rcu_gp_fqs_loop(void)
 			WRITE_ONCE(rcu_state.jiffies_kick_kthreads,
 				   jiffies + (j ? 3 * j : 2));
 		}
-		trace_rcu_grace_period(rcu_state.name,
-				       READ_ONCE(rcu_state.gp_seq),
+		trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 				       TPS("fqswait"));
 		rcu_state.gp_state = RCU_GP_WAIT_FQS;
 		ret = swait_event_idle_timeout_exclusive(
@@ -1671,13 +1671,11 @@ static void rcu_gp_fqs_loop(void)
 		/* If time for quiescent-state forcing, do it. */
 		if (ULONG_CMP_GE(jiffies, rcu_state.jiffies_force_qs) ||
 		    (gf & RCU_GP_FLAG_FQS)) {
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqsstart"));
 			rcu_gp_fqs(first_gp_fqs);
 			first_gp_fqs = false;
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqsend"));
 			cond_resched_tasks_rcu_qs();
 			WRITE_ONCE(rcu_state.gp_activity, jiffies);
@@ -1688,8 +1686,7 @@ static void rcu_gp_fqs_loop(void)
 			cond_resched_tasks_rcu_qs();
 			WRITE_ONCE(rcu_state.gp_activity, jiffies);
 			WARN_ON(signal_pending(current));
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqswaitsig"));
 			ret = 1; /* Keep old FQS timing. */
 			j = jiffies;
@@ -1766,6 +1763,7 @@ static void rcu_gp_cleanup(void)
 	/* Declare grace period done, trace first to use old GP number. */
 	trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("end"));
 	rcu_seq_end(&rcu_state.gp_seq);
+	ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
 	rcu_state.gp_state = RCU_GP_IDLE;
 	/* Check for GP requests since above loop. */
 	rdp = this_cpu_ptr(&rcu_data);
@@ -1781,7 +1779,7 @@ static void rcu_gp_cleanup(void)
 		WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
 		WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
 		trace_rcu_grace_period(rcu_state.name,
-				       READ_ONCE(rcu_state.gp_seq),
+				       rcu_state.gp_seq,
 				       TPS("newreq"));
 	} else {
 		WRITE_ONCE(rcu_state.gp_flags,
@@ -1800,8 +1798,7 @@ static int __noreturn rcu_gp_kthread(void *unused)
 
 		/* Handle grace-period start. */
 		for (;;) {
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("reqwait"));
 			rcu_state.gp_state = RCU_GP_WAIT_GPS;
 			swait_event_idle_exclusive(rcu_state.gp_wq,
@@ -1814,8 +1811,7 @@ static int __noreturn rcu_gp_kthread(void *unused)
 			cond_resched_tasks_rcu_qs();
 			WRITE_ONCE(rcu_state.gp_activity, jiffies);
 			WARN_ON(signal_pending(current));
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("reqwaitsig"));
 		}
 
-- 
2.9.5

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