linux-kernel - Re: [PATCH 2/3] selftests: coredump: Use waitpid() instead of busy-wait

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Message-ID: <84y0wkkrsz.fsf@jogness.linutronix.de>
Date: Tue, 01 Apr 2025 14:14:28 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Nam Cao <namcao@...utronix.de>, Christian Brauner <brauner@...nel.org>
Cc: Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, Nam Cao <namcao@...utronix.de>
Subject: Re: [PATCH 2/3] selftests: coredump: Use waitpid() instead of
 busy-wait

On 2025-03-31, Nam Cao <namcao@...utronix.de> wrote:
> The test waits for coredump to finish by busy-waiting for the
> stackdump_values file to be created. The maximum wait time is 10 seconds.
>
> This doesn't work for slow machine (qemu-system-riscv64), because coredump
> takes longer.
>
> Switch to use waitpid().

Note that you are now assuming that returning from waitpid() means that:

1. the coredumping has completed

and

2. the STACKDUMP_FILE with all its contents are visible to the parent
process

> With this, the stack_values file doesn't need to be atomically written by
> coredump anymore, therefore simplify the stackdump script.
>
> Fixes: 15858da53542 ("selftests: coredump: Add stackdump test")
> Signed-off-by: Nam Cao <namcao@...utronix.de>
> ---
>  tools/testing/selftests/coredump/stackdump        |  6 +-----
>  tools/testing/selftests/coredump/stackdump_test.c | 13 ++++++-------
>  2 files changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/tools/testing/selftests/coredump/stackdump b/tools/testing/selftests/coredump/stackdump
> index 96714ce42d12..ad487fd5ff15 100755
> --- a/tools/testing/selftests/coredump/stackdump
> +++ b/tools/testing/selftests/coredump/stackdump
> @@ -4,11 +4,7 @@
>  CRASH_PROGRAM_ID=$1
>  STACKDUMP_FILE=$2
>  
> -TMP=$(mktemp)
> -
>  for t in /proc/$CRASH_PROGRAM_ID/task/*; do
>  	tid=$(basename $t)
> -	cat /proc/$tid/stat | awk '{print $29}' >> $TMP
> +	cat /proc/$tid/stat | awk '{print $29}' >> $STACKDUMP_FILE
>  done
> -
> -mv $TMP $STACKDUMP_FILE

I would leave this as it was. Then the availability of STACKDUMP_FILE
means the full contents are available.

> diff --git a/tools/testing/selftests/coredump/stackdump_test.c b/tools/testing/selftests/coredump/stackdump_test.c
> index 1dc54e128586..733feaa0f895 100644
> --- a/tools/testing/selftests/coredump/stackdump_test.c
> +++ b/tools/testing/selftests/coredump/stackdump_test.c
> @@ -96,7 +96,7 @@ TEST_F(coredump, stackdump)
>  	char *test_dir, *line;
>  	size_t line_length;
>  	char buf[PATH_MAX];
> -	int ret, i;
> +	int ret, i, status;
>  	FILE *file;
>  	pid_t pid;
>  
> @@ -131,12 +131,11 @@ TEST_F(coredump, stackdump)
>  	/*
>  	 * Step 3: Wait for the stackdump script to write the stack pointers to the stackdump file
>  	 */
> -	for (i = 0; i < 10; ++i) {
> -		file = fopen(STACKDUMP_FILE, "r");
> -		if (file)
> -			break;
> -		sleep(1);
> -	}
> +	waitpid(pid, &status, 0);
> +	ASSERT_TRUE(WIFSIGNALED(status));
> +	ASSERT_TRUE(WCOREDUMP(status));

Why not just put these 3 lines above the for-loop? So you would wait for
the process to end, then go into the 20-second timeout loop waiting for
STACKDUMP_FILE to show up.

John Ogness