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

[Peter Zijlstra]

On Tue, Jul 16, 2019 at 11:10:39AM +0200, Jiri Olsa wrote:
> 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' ?

Thread Sanitizer, from the group that brought us KASAN and UBSAN.

I'd much rather see the below use READ_ONCE() and WRITE_ONCE(). Using
atomic_t without RmW ops is confusing.

> > 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@xxxxxxxxxx>
> > ---
> >  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
> >