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Date:   Tue, 11 Dec 2018 13:45:52 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Thomas Graf <tgraf@suug.ch>,
        "David S. Miller" <davem@davemloft.net>,
        NeilBrown <neilb@suse.com>, Tom Herbert <tom@quantonium.net>,
        Kees Cook <keescook@chromium.org>,
        Paul Blakey <paulb@mellanox.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Phil Sutter <phil@nwl.cc>
Subject: Re: [PATCH] test_rhashtable: remove semaphore usage
Message-ID: <20181211054552.peone47xl5czpfar@gondor.apana.org.au>
References: <20181210211957.210189-1-arnd@arndb.de>
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On Mon, Dec 10, 2018 at 10:17:20PM +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.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.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(-)

This was created by Phil Sutter so I am adding him to the cc list.

> 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
> 

-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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