selftests/damon: add a test for a race between target_ids_read() and dbgfs_before_terminate()

commit 34796417964b ("mm/damon/dbgfs: protect targets destructions with
kdamond_lock") fixed a race of DAMON debugfs interface.  Specifically, the
race was happening between target_ids_read() and dbgfs_before_terminate().
Add a test for the issue to prevent the problem from accidentally
recurring.

Link: https://lkml.kernel.org/r/20240207203134.69976-7-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/.gitignore

@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
 huge_count_read_write
+debugfs_target_ids_read_before_terminate_race

tools/testing/selftests/damon/Makefile

@@ -2,6 +2,7 @@
 # Makefile for damon selftests
 
 TEST_GEN_FILES += huge_count_read_write
+TEST_GEN_FILES += debugfs_target_ids_read_before_terminate_race
 TEST_GEN_FILES += access_memory
 
 TEST_FILES = _chk_dependency.sh _debugfs_common.sh
@@ -9,6 +10,7 @@ TEST_PROGS = debugfs_attrs.sh debugfs_schemes.sh debugfs_target_ids.sh
 TEST_PROGS += debugfs_empty_targets.sh debugfs_huge_count_read_write.sh
 TEST_PROGS += debugfs_duplicate_context_creation.sh
 TEST_PROGS += debugfs_rm_non_contexts.sh
+TEST_PROGS += debugfs_target_ids_read_before_terminate_race.sh
 TEST_PROGS += sysfs.sh sysfs_update_removed_scheme_dir.sh
 TEST_PROGS += sysfs_update_schemes_tried_regions_hang.py
 TEST_PROGS += sysfs_update_schemes_tried_regions_wss_estimation.py

tools/testing/selftests/damon/debugfs_target_ids_read_before_terminate_race.c

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Author: SeongJae Park <sj@kernel.org>
+ */
+#define _GNU_SOURCE
+
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <time.h>
+#include <unistd.h>
+
+#define DBGFS_MONITOR_ON "/sys/kernel/debug/damon/monitor_on_DEPRECATED"
+#define DBGFS_TARGET_IDS "/sys/kernel/debug/damon/target_ids"
+
+static void turn_damon_on_exit(void)
+{
+	int target_ids_fd = open(DBGFS_TARGET_IDS, O_RDWR);
+	int monitor_on_fd = open(DBGFS_MONITOR_ON, O_RDWR);
+	char pid_str[128];
+
+	snprintf(pid_str, sizeof(pid_str), "%d", getpid());
+	write(target_ids_fd, pid_str, sizeof(pid_str));
+	write(monitor_on_fd, "on\n", 3);
+	close(target_ids_fd);
+	close(monitor_on_fd);
+	usleep(1000);
+	exit(0);
+}
+
+static void try_race(void)
+{
+	int target_ids_fd = open(DBGFS_TARGET_IDS, O_RDWR);
+	int pid = fork();
+	int buf[256];
+
+	if (pid < 0) {
+		fprintf(stderr, "fork() failed\n");
+		exit(1);
+	}
+	if (pid == 0)
+		turn_damon_on_exit();
+	while (true) {
+		int status;
+
+		read(target_ids_fd, buf, sizeof(buf));
+		if (waitpid(-1, &status, WNOHANG) == pid)
+			break;
+	}
+	close(target_ids_fd);
+}
+
+static inline uint64_t ts_to_ms(struct timespec *ts)
+{
+	return (uint64_t)ts->tv_sec * 1000 + (uint64_t)ts->tv_nsec / 1000000;
+}
+
+int main(int argc, char *argv[])
+{
+	struct timespec start_time, now;
+	int runtime_ms;
+
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s <runtime in ms>\n", argv[0]);
+		exit(1);
+	}
+	runtime_ms = atoi(argv[1]);
+	clock_gettime(CLOCK_MONOTONIC, &start_time);
+	while (true) {
+		try_race();
+		clock_gettime(CLOCK_MONOTONIC, &now);
+		if (ts_to_ms(&now) - ts_to_ms(&start_time) > runtime_ms)
+			break;
+	}
+	return 0;
+}

tools/testing/selftests/damon/debugfs_target_ids_read_before_terminate_race.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+dmesg -C
+
+./debugfs_target_ids_read_before_terminate_race 5000
+
+if dmesg | grep -q dbgfs_target_ids_read
+then
+	dmesg
+	exit 1
+else
+	exit 0
+fi