devpts: Convert to new IDA API

ida_alloc_max() matches what this driver wants to do.  Also removes a
call to ida_pre_get().  We no longer need the protection of the mutex,
so convert pty_count to an atomic_t and remove the mutex entirely.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

fs/devpts/inode.c

@@ -46,7 +46,7 @@ static int pty_limit = NR_UNIX98_PTY_DEFAULT;
 static int pty_reserve = NR_UNIX98_PTY_RESERVE;
 static int pty_limit_min;
 static int pty_limit_max = INT_MAX;
-static int pty_count;
+static atomic_t pty_count = ATOMIC_INIT(0);
 
 static struct ctl_table pty_table[] = {
 	{
@@ -93,8 +93,6 @@ static struct ctl_table pty_root_table[] = {
 	{}
 };
 
-static DEFINE_MUTEX(allocated_ptys_lock);
-
 struct pts_mount_opts {
 	int setuid;
 	int setgid;
@@ -533,44 +531,25 @@ static struct file_system_type devpts_fs_type = {
 
 int devpts_new_index(struct pts_fs_info *fsi)
 {
-	int index;
-	int ida_ret;
-
-retry:
-	if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
-		return -ENOMEM;
-
-	mutex_lock(&allocated_ptys_lock);
-	if (pty_count >= (pty_limit -
-			  (fsi->mount_opts.reserve ? 0 : pty_reserve))) {
-		mutex_unlock(&allocated_ptys_lock);
-		return -ENOSPC;
-	}
+	int index = -ENOSPC;
 
-	ida_ret = ida_get_new(&fsi->allocated_ptys, &index);
-	if (ida_ret < 0) {
-		mutex_unlock(&allocated_ptys_lock);
-		if (ida_ret == -EAGAIN)
-			goto retry;
-		return -EIO;
-	}
+	if (atomic_inc_return(&pty_count) >= (pty_limit -
+			  (fsi->mount_opts.reserve ? 0 : pty_reserve)))
+		goto out;
 
-	if (index >= fsi->mount_opts.max) {
-		ida_remove(&fsi->allocated_ptys, index);
-		mutex_unlock(&allocated_ptys_lock);
-		return -ENOSPC;
-	}
-	pty_count++;
-	mutex_unlock(&allocated_ptys_lock);
+	index = ida_alloc_max(&fsi->allocated_ptys, fsi->mount_opts.max - 1,
+			GFP_KERNEL);
+
+out:
+	if (index < 0)
+		atomic_dec(&pty_count);
 	return index;
 }
 
 void devpts_kill_index(struct pts_fs_info *fsi, int idx)
 {
-	mutex_lock(&allocated_ptys_lock);
-	ida_remove(&fsi->allocated_ptys, idx);
-	pty_count--;
-	mutex_unlock(&allocated_ptys_lock);
+	ida_free(&fsi->allocated_ptys, idx);
+	atomic_dec(&pty_count);
 }
 
 /**