[PATCH v4 2/3] perf sched stats: Fix SIGCHLD vs pause() race in schedstat_live()

[Swapnil Sapkal <swapnil.sapkal@amd.com>]

perf_sched__schedstat_live() has the same lost-wakeup race as
perf_sched__schedstat_record(): a short-lived workload's SIGCHLD
can be consumed by the signal handler before pause() is entered,
hanging the process.

Apply the same fix: replace pause() with a loop checking the 'done'
flag and using waitpid(WNOHANG) for the workload case.

Suggested-by: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@amd.com>
---
 tools/perf/builtin-sched.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 7da71c372e25..9e8941ae1b3d 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -4755,6 +4755,7 @@ static int perf_sched__schedstat_live(struct perf_sched *sched,
 	int reset = 0;
 	int err = 0;
 
+	done = 0;
 	signal(SIGINT, sighandler);
 	signal(SIGCHLD, sighandler);
 	signal(SIGTERM, sighandler);
@@ -4800,8 +4801,11 @@ static int perf_sched__schedstat_live(struct perf_sched *sched,
 	if (argc)
 		evlist__start_workload(evlist);
 
-	/* wait for signal */
-	pause();
+	while (!done) {
+		if (argc && waitpid(evlist->workload.pid, NULL, WNOHANG) > 0)
+			break;
+		sleep(1);
+	}
 
 	if (reset) {
 		err = disable_sched_schedstat();
-- 
2.43.0