[PATCH v4 3/3] perf lock contention: Fix SIGCHLD vs pause() race in __cmd_contention()

[Swapnil Sapkal]

__cmd_contention() suffers from the same lost-wakeup race as the perf
sched stats paths: 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@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@xxxxxxx>
---
 tools/perf/builtin-lock.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
index 5585aeb97684..e1c033e3a8d5 100644
--- a/tools/perf/builtin-lock.c
+++ b/tools/perf/builtin-lock.c
@@ -31,6 +31,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 #include <sys/prctl.h>
+#include <sys/wait.h>
 #include <semaphore.h>
 #include <math.h>
 #include <limits.h>
@@ -1921,8 +1922,11 @@ static int __cmd_report(bool display_info)
 	return err;
 }
 
+static volatile sig_atomic_t done;
+
 static void sighandler(int sig __maybe_unused)
 {
+	done = 1;
 }
 
 static int check_lock_contention_options(const struct option *options,
@@ -2060,6 +2064,7 @@ static int __cmd_contention(int argc, const char **argv)
 			goto out_delete;
 		}
 
+		done = 0;
 		signal(SIGINT, sighandler);
 		signal(SIGCHLD, sighandler);
 		signal(SIGTERM, sighandler);
@@ -2127,8 +2132,11 @@ static int __cmd_contention(int argc, const char **argv)
 		if (argc)
 			evlist__start_workload(con.evlist);
 
-		/* wait for signal */
-		pause();
+		while (!done) {
+			if (argc && waitpid(con.evlist->workload.pid, NULL, WNOHANG) > 0)
+				break;
+			sleep(1);
+		}
 
 		lock_contention_stop();
 		lock_contention_read(&con);
-- 
2.43.0