linux-kernel - [PATCH tip/core/rcu 14/30] refperf: Convert reader

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Message-Id: <20200623003013.26252-14-paulmck@kernel.org>
Date:   Mon, 22 Jun 2020 17:29:57 -0700
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 14/30] refperf: Convert reader_task structure's "start" field to int

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

This commit converts the reader_task structure's "start" field to int
in order to demote a full barrier to an smp_load_acquire() and also to
simplify the code a bit.  While in the area, and to enlist the compiler's
help in ensuring that nothing was missed, the field's name was changed
to start_reader.

Also while in the area, change the main_func() store to use
smp_store_release() to further fortify against wait/wake races.

Cc: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/refperf.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/refperf.c b/kernel/rcu/refperf.c
index 8815ccf..2fd3ed1 100644
--- a/kernel/rcu/refperf.c
+++ b/kernel/rcu/refperf.c
@@ -80,7 +80,7 @@ torture_param(bool, shutdown, REFPERF_SHUTDOWN,
 
 struct reader_task {
 	struct task_struct *task;
-	atomic_t start;
+	int start_reader;
 	wait_queue_head_t wq;
 	u64 last_duration_ns;
 };
@@ -243,7 +243,7 @@ ref_perf_reader(void *arg)
 	VERBOSE_PERFOUT("ref_perf_reader %ld: waiting to start next experiment on cpu %d", me, smp_processor_id());
 
 	// Wait for signal that this reader can start.
-	wait_event(rt->wq, (atomic_read(&nreaders_exp) && atomic_read(&rt->start)) ||
+	wait_event(rt->wq, (atomic_read(&nreaders_exp) && smp_load_acquire(&rt->start_reader)) ||
 			   torture_must_stop());
 
 	if (torture_must_stop())
@@ -252,8 +252,7 @@ ref_perf_reader(void *arg)
 	// Make sure that the CPU is affinitized appropriately during testing.
 	WARN_ON_ONCE(smp_processor_id() != me);
 
-	smp_mb__before_atomic();
-	atomic_dec(&rt->start);
+	WRITE_ONCE(rt->start_reader, 0);
 
 	VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d started", me, exp_idx);
 
@@ -372,7 +371,7 @@ static int main_func(void *arg)
 		exp_idx = exp;
 
 		for (r = 0; r < nreaders; r++) {
-			atomic_set(&reader_tasks[r].start, 1);
+			smp_store_release(&reader_tasks[r].start_reader, 1);
 			wake_up(&reader_tasks[r].wq);
 		}
 
-- 
2.9.5