[PATCH] [v2] test_rhashtable: remove semaphore usage

[PATCH] [v2] test_rhashtable: remove semaphore usage

[Arnd Bergmann]

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.

Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
Changes from v1:
 - rebase to mainline,
 - fix pr_err() output
 - bail out if interrupted
---
 lib/test_rhashtable.c | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/lib/test_rhashtable.c b/lib/test_rhashtable.c
index 82ac39ce5310..6a8ac7626797 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
@@ -112,8 +112,8 @@ static struct rhashtable_params test_rht_params_dup = {
 	.automatic_shrinking = false,
 };
 
-static struct semaphore prestart_sem;
-static struct semaphore startup_sem = __SEMAPHORE_INITIALIZER(startup_sem, 0);
+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)
@@ -634,9 +634,12 @@ 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))
-		pr_err("  thread[%d]: down_interruptible failed\n", tdata->id);
+	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]: interrupted\n", tdata->id);
+		goto out;
+	}
 
 	for (i = 0; i < tdata->entries; i++) {
 		tdata->objs[i].value.id = i;
@@ -755,7 +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);
+	atomic_set(&startup_count, tcount);
 	tdata = vzalloc(array_size(tcount, sizeof(struct thread_data)));
 	if (!tdata)
 		return -ENOMEM;
@@ -781,15 +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

Re: [PATCH] [v2] test_rhashtable: remove semaphore usage

[Herbert Xu]

On Sun, Dec 16, 2018 at 08:48:21PM +0100, Arnd Bergmann wrote:
> 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.
> 
> Acked-by: Phil Sutter <phil@nwl.cc>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] [v2] test_rhashtable: remove semaphore usage

[David Miller]

From: Arnd Bergmann <arnd@arndb.de>
Date: Sun, 16 Dec 2018 20:48:21 +0100

> 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.
> 
> Acked-by: Phil Sutter <phil@nwl.cc>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> Changes from v1:
>  - rebase to mainline,
>  - fix pr_err() output
>  - bail out if interrupted

Applied, thanks Arnd.

Re: [PATCH] [v2] test_rhashtable: remove semaphore usage

[Geert Uytterhoeven]

Hi Arnd,

On Sun, Dec 16, 2018 at 8:50 PM Arnd Bergmann <arnd@arndb.de> wrote:
> 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.
>
> Acked-by: Phil Sutter <phil@nwl.cc>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> Changes from v1:
>  - rebase to mainline,
>  - fix pr_err() output
>  - bail out if interrupted

Now this is upstream as 809c67059162e7ba ("test_rhashtable: remove
semaphore usage"), I gave test_rhashtable a try again on m68k/ARAnyM.
And it succeeded, without my workaround from
https://www.mail-archive.com/netdev@vger.kernel.org/msg134240.html
Interestingly, it still succeeded without the workaround after reverting
809c67059162e7ba, so some other change during the last two years must
have fixed this.

I have no plans to bisect this, though.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds