linux-kernel - Re: [PATCH] Fix perf test data race for tsan build

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Date:   Tue, 16 Jul 2019 11:10:39 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Numfor Mbiziwo-Tiapo <nums@...gle.com>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, namhyung@...nel.org,
        songliubraving@...com, mbd@...com, linux-kernel@...r.kernel.org,
        irogers@...gle.com, eranian@...gle.com
Subject: Re: [PATCH] Fix perf test data race for tsan build

On Wed, Jul 10, 2019 at 03:16:20PM -0700, Numfor Mbiziwo-Tiapo wrote:
> The tsan build of perf gives a data race warning in

hi,
what's 'tsan build of perf' ?

thanks,
jirka


> mmap-thread-lookup.c when perf test is run on our local machines.
> This is because there is no lock around the "go_away" variable in
> "mmap-thread-lookup.c".
> 
> The warning is difficult to reproduce from the tip branch and we
> suspect it has something to do with tsan working differently
> depending on the clang version.
> 
> The warning can be silenced by adding locks around the variable
> but it is better practice to make the variable atomic and use
> the atomic_set and atomic_read functions. This does not actually
> silence the warning because tsan doesn't recognize the atomic
> functions as thread safe, but in actuality it should prevent a
> data race.
> 
> Signed-off-by: Numfor Mbiziwo-Tiapo <nums@...gle.com>
> ---
>  tools/perf/tests/mmap-thread-lookup.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/tests/mmap-thread-lookup.c b/tools/perf/tests/mmap-thread-lookup.c
> index 5ede9b561d32..f4cbb3d92a46 100644
> --- a/tools/perf/tests/mmap-thread-lookup.c
> +++ b/tools/perf/tests/mmap-thread-lookup.c
> @@ -17,7 +17,7 @@
>  
>  #define THREADS 4
>  
> -static int go_away;
> +static atomic_t go_away;
>  
>  struct thread_data {
>  	pthread_t	pt;
> @@ -64,7 +64,7 @@ static void *thread_fn(void *arg)
>  		return NULL;
>  	}
>  
> -	while (!go_away) {
> +	while (!atomic_read(&go_away)) {
>  		/* Waiting for main thread to kill us. */
>  		usleep(100);
>  	}
> @@ -98,7 +98,7 @@ static int threads_create(void)
>  	struct thread_data *td0 = &threads[0];
>  	int i, err = 0;
>  
> -	go_away = 0;
> +	atomic_set(&go_away, 0);
>  
>  	/* 0 is main thread */
>  	if (thread_init(td0))
> @@ -118,7 +118,7 @@ static int threads_destroy(void)
>  	/* cleanup the main thread */
>  	munmap(td0->map, page_size);
>  
> -	go_away = 1;
> +	atomic_set(&go_away, 1);
>  
>  	for (i = 1; !err && i < THREADS; i++)
>  		err = pthread_join(threads[i].pt, NULL);
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
>