[PATCH 1/3] perf sched stats: Fix SIGCHLD race in schedstat_record()

[Swapnil Sapkal]

When a very short-lived workload is used with 'perf sched stats record',
the child process can exit and deliver SIGCHLD between
evlist__start_workload() and pause(). Since pause() only returns when a
signal is received while suspended, and the SIGCHLD has already been
delivered and handled by then, pause() blocks indefinitely.

Fix this by blocking SIGCHLD before starting the workload and replacing
pause() with sigsuspend(). sigsuspend() atomically unblocks SIGCHLD and
suspends the process, ensuring no signal is lost regardless of how
quickly the child exits.

Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@xxxxxxx>
---
 tools/perf/builtin-sched.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 3f509cfdd58c..eb3702d98fd1 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -3807,6 +3807,7 @@ const char *output_name;
 static int perf_sched__schedstat_record(struct perf_sched *sched,
 					int argc, const char **argv)
 {
+	sigset_t sigchld_mask, oldmask;
 	struct perf_session *session;
 	struct target target = {};
 	struct evlist *evlist;
@@ -3822,6 +3823,15 @@ static int perf_sched__schedstat_record(struct perf_sched *sched,
 	signal(SIGCHLD, sighandler);
 	signal(SIGTERM, sighandler);
 
+	/*
+	 * Block SIGCHLD early so that a short-lived workload cannot deliver
+	 * the signal before we are ready to wait for it. sigsuspend() below
+	 * will atomically unblock it.
+	 */
+	sigemptyset(&sigchld_mask);
+	sigaddset(&sigchld_mask, SIGCHLD);
+	sigprocmask(SIG_BLOCK, &sigchld_mask, &oldmask);
+
 	evlist = evlist__new();
 	if (!evlist)
 		return -ENOMEM;
@@ -3902,8 +3912,15 @@ static int perf_sched__schedstat_record(struct perf_sched *sched,
 	if (argc)
 		evlist__start_workload(evlist);
 
-	/* wait for signal */
-	pause();
+	/*
+	 * Use sigsuspend() instead of pause() to avoid a race where a
+	 * short-lived workload exits and delivers SIGCHLD before pause()
+	 * is entered, causing it to block indefinitely. sigsuspend()
+	 * atomically unblocks SIGCHLD (blocked above) and suspends,
+	 * ensuring no signal is lost.
+	 */
+	sigsuspend(&oldmask);
+	sigprocmask(SIG_SETMASK, &oldmask, NULL);
 
 	if (reset) {
 		err = disable_sched_schedstat();
-- 
2.43.0