[PATCH v7] perf tpebs: Fix concurrent stop races and PID reuse hazards in tpebs_stop

[Ian Rogers <irogers@google.com>]

Parallel verbose test execution can trigger a race condition in tpebs_stop
if called concurrently or when PID reuse occurs, causing finish_command()
to block or reap the wrong process.

Introduce a `tpebs_stopping` flag inside intel-tpebs.c to prevent
redundant stop execution paths, and safely restore the `cmd.pid`
temporarily only during `finish_command()` to ensure it is properly reaped,
while preventing other threads from referencing it.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/intel-tpebs.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/perf/util/intel-tpebs.c b/tools/perf/util/intel-tpebs.c
index ed8cfe2ba2fa..ddb0579a451e 100644
--- a/tools/perf/util/intel-tpebs.c
+++ b/tools/perf/util/intel-tpebs.c
@@ -37,6 +37,7 @@ static pthread_t tpebs_reader_thread;
 static struct child_process tpebs_cmd;
 static int control_fd[2], ack_fd[2];
 static struct mutex tpebs_mtx;
+static bool tpebs_stopping;
 
 struct tpebs_retire_lat {
 	struct list_head nd;
@@ -320,8 +321,14 @@ static int tpebs_stop(void) EXCLUSIVE_LOCKS_REQUIRED(tpebs_mtx_get())
 {
 	int ret = 0;
 
+	if (tpebs_stopping)
+		return 0;
+
 	/* Like tpebs_start, we should only run tpebs_end once. */
 	if (tpebs_cmd.pid != 0) {
+		pid_t actual_pid = tpebs_cmd.pid;
+
+		tpebs_stopping = true;
 		tpebs_send_record_cmd(EVLIST_CTL_CMD_STOP_TAG);
 		tpebs_cmd.pid = 0;
 		mutex_unlock(tpebs_mtx_get());
@@ -332,8 +339,10 @@ static int tpebs_stop(void) EXCLUSIVE_LOCKS_REQUIRED(tpebs_mtx_get())
 		close(ack_fd[0]);
 		close(ack_fd[1]);
 		close(tpebs_cmd.out);
+		tpebs_cmd.pid = actual_pid;
 		ret = finish_command(&tpebs_cmd);
 		tpebs_cmd.pid = 0;
+		tpebs_stopping = false;
 		if (ret == -ERR_RUN_COMMAND_WAITPID_SIGNAL)
 			ret = 0;
 	}
-- 
2.54.0.929.g9b7fa37559-goog