linux-kernel - [PATCH] test_rhashtable: remove semaphore usage

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Message-Id: <20181210211957.210189-1-arnd@arndb.de>
Date:   Mon, 10 Dec 2018 22:17:20 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Thomas Graf <tgraf@...g.ch>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        NeilBrown <neilb@...e.com>, Tom Herbert <tom@...ntonium.net>,
        Kees Cook <keescook@...omium.org>,
        Paul Blakey <paulb@...lanox.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] test_rhashtable: remove semaphore usage

This is one of only two files that initialize a semaphore to a negative
value. We don't really need the two semaphores here at all, but can do
the same thing in more conventional and more effient way, by using a
single waitqueue and an atomic thread counter.

This gets us a little bit closer to eliminating classic semaphores from
the kernel. It also fixes a corner case where we fail to continue after
one of the threads fails to start up.

An alternative would be to use a split kthread_create()+wake_up_process()
and completely eliminate the separate synchronization.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
This is part of a longer, untested, series to remove semaphores
from the kernel, please review as such before applying.
---
 lib/test_rhashtable.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/lib/test_rhashtable.c b/lib/test_rhashtable.c
index 18de5ff1255b..12bdea4f6c20 100644
--- a/lib/test_rhashtable.c
+++ b/lib/test_rhashtable.c
@@ -20,11 +20,11 @@
 #include <linux/module.h>
 #include <linux/rcupdate.h>
 #include <linux/rhashtable.h>
-#include <linux/semaphore.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
 #include <linux/random.h>
 #include <linux/vmalloc.h>
+#include <linux/wait.h>
 
 #define MAX_ENTRIES	1000000
 #define TEST_INSERT_FAIL INT_MAX
@@ -112,7 +112,8 @@ static struct rhashtable_params test_rht_params_dup = {
 	.automatic_shrinking = false,
 };
 
-static struct semaphore prestart_sem, startup_sem;
+static atomic_t startup_count;
+static DECLARE_WAIT_QUEUE_HEAD(startup_wait);
 
 static int insert_retry(struct rhashtable *ht, struct test_obj *obj,
                         const struct rhashtable_params params)
@@ -635,8 +636,9 @@ static int threadfunc(void *data)
 	int i, step, err = 0, insert_retries = 0;
 	struct thread_data *tdata = data;
 
-	up(&prestart_sem);
-	if (down_interruptible(&startup_sem))
+	if (atomic_dec_and_test(&startup_count))
+		wake_up(&startup_wait);
+	if (wait_event_interruptible(startup_wait, atomic_read(&startup_count) == -1))
 		pr_err("  thread[%d]: down_interruptible failed\n", tdata->id);
 
 	for (i = 0; i < tdata->entries; i++) {
@@ -756,8 +758,7 @@ static int __init test_rht_init(void)
 
 	pr_info("Testing concurrent rhashtable access from %d threads\n",
 	        tcount);
-	sema_init(&prestart_sem, 1 - tcount);
-	sema_init(&startup_sem, 0);
+	atomic_set(&startup_count, tcount);
 	tdata = vzalloc(array_size(tcount, sizeof(struct thread_data)));
 	if (!tdata)
 		return -ENOMEM;
@@ -783,15 +784,18 @@ static int __init test_rht_init(void)
 		tdata[i].objs = objs + i * entries;
 		tdata[i].task = kthread_run(threadfunc, &tdata[i],
 		                            "rhashtable_thrad[%d]", i);
-		if (IS_ERR(tdata[i].task))
+		if (IS_ERR(tdata[i].task)) {
 			pr_err(" kthread_run failed for thread %d\n", i);
-		else
+			atomic_dec(&startup_count);
+		} else {
 			started_threads++;
+		}
 	}
-	if (down_interruptible(&prestart_sem))
-		pr_err("  down interruptible failed\n");
-	for (i = 0; i < tcount; i++)
-		up(&startup_sem);
+	if (wait_event_interruptible(startup_wait, atomic_read(&startup_count) == 0))
+		pr_err("  wait_event interruptible failed\n");
+	/* count is 0 now, set it to -1 and wake up all threads together */
+	atomic_dec(&startup_count);
+	wake_up_all(&startup_wait);
 	for (i = 0; i < tcount; i++) {
 		if (IS_ERR(tdata[i].task))
 			continue;
-- 
2.20.0