mm/damon/dbgfs: fix missed use of damon_dbgfs_lock

DAMON debugfs is supposed to protect dbgfs_ctxs, dbgfs_nr_ctxs, and
dbgfs_dirs using damon_dbgfs_lock.  However, some of the code is
accessing the variables without the protection.  This fixes it by
protecting all such accesses.

Link: https://lkml.kernel.org/r/20211110145758.16558-3-sj@kernel.org
Fixes: 75c1c2b5 ("mm/damon/dbgfs: support multiple contexts")
Signed-off-by: NSeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/damon/dbgfs.c

@@ -877,12 +877,14 @@ static ssize_t dbgfs_monitor_on_write(struct file *file,
 		return -EINVAL;
 	}
 
+	mutex_lock(&damon_dbgfs_lock);
 	if (!strncmp(kbuf, "on", count)) {
 		int i;
 
 		for (i = 0; i < dbgfs_nr_ctxs; i++) {
 			if (damon_targets_empty(dbgfs_ctxs[i])) {
 				kfree(kbuf);
+				mutex_unlock(&damon_dbgfs_lock);
 				return -EINVAL;
 			}
 		}
@@ -892,6 +894,7 @@ static ssize_t dbgfs_monitor_on_write(struct file *file,
 	} else {
 		ret = -EINVAL;
 	}
+	mutex_unlock(&damon_dbgfs_lock);
 
 	if (!ret)
 		ret = count;
@@ -944,15 +947,16 @@ static int __init __damon_dbgfs_init(void)
 
 static int __init damon_dbgfs_init(void)
 {
-	int rc;
+	int rc = -ENOMEM;
 
+	mutex_lock(&damon_dbgfs_lock);
 	dbgfs_ctxs = kmalloc(sizeof(*dbgfs_ctxs), GFP_KERNEL);
 	if (!dbgfs_ctxs)
-		return -ENOMEM;
+		goto out;
 	dbgfs_ctxs[0] = dbgfs_new_ctx();
 	if (!dbgfs_ctxs[0]) {
 		kfree(dbgfs_ctxs);
-		return -ENOMEM;
+		goto out;
 	}
 	dbgfs_nr_ctxs = 1;
 
@@ -963,6 +967,8 @@ static int __init damon_dbgfs_init(void)
 		pr_err("%s: dbgfs init failed\n", __func__);
 	}
 
+out:
+	mutex_unlock(&damon_dbgfs_lock);
 	return rc;
 }