init: flush async file closing

When unpacking the initramfs or when mounting block devices we need to
ensure that any delayed fput() finished to prevent spurious errors.
The init process can be a proper kernel thread or a user mode helper.
In the latter case PF_KTHREAD isn't set. So we need to do both
flush_delayed_work() and task_work_run().

Since we'll port block device opening and closing to regular file open
and closing we need to ensure the same as for the initramfs. So just
make that a little helper.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Srikanth Aithal <sraithal@amd.com>
Link: https://lore.kernel.org/r/CA+G9fYttTwsbFuVq10igbSvP5xC6bf_XijM=mpUqrJV=uvUirQ@mail.gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

init/do_mounts.c

@@ -208,6 +208,9 @@ retry:
 				goto out;
 			case -EACCES:
 			case -EINVAL:
+#ifdef CONFIG_BLOCK
+				init_flush_fput();
+#endif
 				continue;
 		}
 	        /*

init/do_mounts.h

@@ -9,6 +9,8 @@
 #include <linux/major.h>
 #include <linux/root_dev.h>
 #include <linux/init_syscalls.h>
+#include <linux/task_work.h>
+#include <linux/file.h>
 
 void  mount_root_generic(char *name, char *pretty_name, int flags);
 void  mount_root(char *root_device_name);
@@ -41,3 +43,10 @@ static inline bool initrd_load(char *root_device_name)
 	}
 
 #endif
+
+/* Ensure that async file closing finished to prevent spurious errors. */
+static inline void init_flush_fput(void)
+{
+	flush_delayed_fput();
+	task_work_run();
+}

init/initramfs.c

@@ -16,9 +16,10 @@
 #include <linux/mm.h>
 #include <linux/namei.h>
 #include <linux/init_syscalls.h>
-#include <linux/task_work.h>
 #include <linux/umh.h>
 
+#include "do_mounts.h"
+
 static __initdata bool csum_present;
 static __initdata u32 io_csum;
 
@@ -736,8 +737,7 @@ done:
 	initrd_start = 0;
 	initrd_end = 0;
 
-	flush_delayed_fput();
-	task_work_run();
+	init_flush_fput();
 }
 
 static ASYNC_DOMAIN_EXCLUSIVE(initramfs_domain);