linux-kernel - Re: [PATCH] selftests/futex: Order calls in futex

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Message-ID: <0f9ec70e-2faf-4bb9-ae79-6cea8f4c3a66@collabora.com>
Date: Fri, 21 Jun 2024 10:45:48 +0500
From: Muhammad Usama Anjum <usama.anjum@...labora.com>
To: Edward Liaw <edliaw@...gle.com>, shuah@...nel.org,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Darren Hart <dvhart@...radead.org>,
 Davidlohr Bueso <dave@...olabs.net>, André Almeida
 <andrealmeid@...lia.com>
Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 kernel-team@...roid.com
Subject: Re: [PATCH] selftests/futex: Order calls in futex_requeue

On 6/19/24 5:22 AM, Edward Liaw wrote:
> Like fbf4dec70277 ("selftests/futex: Order calls to futex_lock_pi"),
> which fixed a flake in futex_lock_pi due to racing between the parent
> and child threads.
> 
> The same issue can occur in the futex_requeue test, because it expects
> waiterfn to make progress to futex_wait before the parent starts to
> requeue. This is mitigated by the parent sleeping for WAKE_WAIT_US, but
> it still fails occasionally. This can be reproduced by adding a sleep in
> the waiterfn before futex_wait:
> 
> TAP version 13
> 1..2
> not ok 1 futex_requeue simple returned: 0
> not ok 2 futex_requeue simple returned: 0
> not ok 3 futex_requeue many returned: 0
> not ok 4 futex_requeue many returned: 0
> 
> Instead, replace the sleep with barriers to make the sequencing
> explicit.
> 
> Fixes: 7cb5dd8e2c8c ("selftests: futex: Add futex compare requeue test")
> Signed-off-by: Edward Liaw <edliaw@...gle.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@...labora.com>

> ---
>  .../selftests/futex/functional/futex_requeue.c       | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/futex/functional/futex_requeue.c b/tools/testing/selftests/futex/functional/futex_requeue.c
> index 51485be6eb2f..8f7d3e8bf32a 100644
> --- a/tools/testing/selftests/futex/functional/futex_requeue.c
> +++ b/tools/testing/selftests/futex/functional/futex_requeue.c
> @@ -12,9 +12,9 @@
>  
>  #define TEST_NAME "futex-requeue"
>  #define timeout_ns  30000000
> -#define WAKE_WAIT_US 10000
>  
>  volatile futex_t *f1;
> +static pthread_barrier_t barrier;
>  
>  void usage(char *prog)
>  {
> @@ -32,6 +32,8 @@ void *waiterfn(void *arg)
>  	to.tv_sec = 0;
>  	to.tv_nsec = timeout_ns;
>  
> +	pthread_barrier_wait(&barrier);
> +
>  	if (futex_wait(f1, *f1, &to, 0))
>  		printf("waiter failed errno %d\n", errno);
>  
> @@ -70,13 +72,15 @@ int main(int argc, char *argv[])
>  	ksft_print_msg("%s: Test futex_requeue\n",
>  		       basename(argv[0]));
>  
> +	pthread_barrier_init(&barrier, NULL, 2);
>  	/*
>  	 * Requeue a waiter from f1 to f2, and wake f2.
>  	 */
>  	if (pthread_create(&waiter[0], NULL, waiterfn, NULL))
>  		error("pthread_create failed\n", errno);
>  
> -	usleep(WAKE_WAIT_US);
> +	pthread_barrier_wait(&barrier);
> +	pthread_barrier_destroy(&barrier);
>  
>  	info("Requeuing 1 futex from f1 to f2\n");
>  	res = futex_cmp_requeue(f1, 0, &f2, 0, 1, 0);
> @@ -99,6 +103,7 @@ int main(int argc, char *argv[])
>  		ksft_test_result_pass("futex_requeue simple succeeds\n");
>  	}
>  
> +	pthread_barrier_init(&barrier, NULL, 11);
>  
>  	/*
>  	 * Create 10 waiters at f1. At futex_requeue, wake 3 and requeue 7.
> @@ -109,7 +114,8 @@ int main(int argc, char *argv[])
>  			error("pthread_create failed\n", errno);
>  	}
>  
> -	usleep(WAKE_WAIT_US);
> +	pthread_barrier_wait(&barrier);
> +	pthread_barrier_destroy(&barrier);
>  
>  	info("Waking 3 futexes at f1 and requeuing 7 futexes from f1 to f2\n");
>  	res = futex_cmp_requeue(f1, 0, &f2, 3, 7, 0);

-- 
BR,
Muhammad Usama Anjum