selftests/bpf: Fix sample_cnt shared between two threads

Make sample_cnt volatile to fix possible selftests failure due to compiler
optimization preventing latest sample_cnt value to be visible to main thread.
sample_cnt is incremented in background thread, which is then joined into main
thread. So in terms of visibility sample_cnt update is ok. But because it's
not volatile, compiler might make optimizations that would prevent main thread
to see latest updated value. Fix this by marking global variable volatile.

Fixes: b7d03ba86d3a ("selftests/bpf: Add BPF ringbuf selftests")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200602050349.215037-1-andriin@fb.com

diff --git a/tools/testing/selftests/bpf/prog_tests/ringbuf.c b/tools/testing/selftests/bpf/prog_tests/ringbuf.c
index bb8541f240e2a0694d6d752c9fe22e49e9ce523a..2bba908dfa6378001c161da1c3efce357476ee0d 100644 (file)
--- a/tools/testing/selftests/bpf/prog_tests/ringbuf.c
+++ b/tools/testing/selftests/bpf/prog_tests/ringbuf.c
@@ -25,7 +25,7 @@ struct sample {
        char comm[16];
 };
 
-static int sample_cnt;
+static volatile int sample_cnt;
 
 static int process_sample(void *ctx, void *data, size_t len)
 {