fs: add ksys_sync() helper; remove in-kernel calls to sys_sync()

Using this helper allows us to avoid the in-kernel calls to the
sys_sync() syscall. The ksys_ prefix denotes that this function
is meant as a drop-in replacement for the syscall. In particular, it
uses the same calling convention as sys_sync().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/sparc/kernel/setup_32.c

@@ -86,7 +86,7 @@ static void prom_sync_me(void)
 	show_free_areas(0, NULL);
 	if (!is_idle_task(current)) {
 		local_irq_enable();
-		sys_sync();
+		ksys_sync();
 		local_irq_disable();
 	}
 	prom_printf("Returning to prom\n");

drivers/tty/sysrq.c

@@ -660,7 +660,7 @@ static void sysrq_do_reset(struct timer_list *t)
 
 	state->reset_requested = true;
 
-	sys_sync();
+	ksys_sync();
 	kernel_restart(NULL);
 }
 

fs/sync.c

@@ -105,7 +105,7 @@ static void fdatawait_one_bdev(struct block_device *bdev, void *arg)
  * just write metadata (such as inodes or bitmaps) to block device page cache
  * and do not sync it on their own in ->sync_fs().
  */
-SYSCALL_DEFINE0(sync)
+void ksys_sync(void)
 {
 	int nowait = 0, wait = 1;
 
@@ -117,6 +117,11 @@ SYSCALL_DEFINE0(sync)
 	iterate_bdevs(fdatawait_one_bdev, NULL);
 	if (unlikely(laptop_mode))
 		laptop_sync_completion();
+}
+
+SYSCALL_DEFINE0(sync)
+{
+	ksys_sync();
 	return 0;
 }
 

include/linux/syscalls.h

@@ -960,6 +960,7 @@ int ksys_getdents64(unsigned int fd, struct linux_dirent64 __user *dirent,
 int ksys_ioctl(unsigned int fd, unsigned int cmd, unsigned long arg);
 off_t ksys_lseek(unsigned int fd, off_t offset, unsigned int whence);
 ssize_t ksys_read(unsigned int fd, char __user *buf, size_t count);
+void ksys_sync(void);
 
 /*
  * The following kernel syscall equivalents are just wrappers to fs-internal

kernel/power/hibernate.c

@@ -701,7 +701,7 @@ int hibernate(void)
 	}
 
 	pr_info("Syncing filesystems ... \n");
-	sys_sync();
+	ksys_sync();
 	pr_info("done.\n");
 
 	error = freeze_processes();

kernel/power/suspend.c

@@ -560,7 +560,7 @@ static int enter_state(suspend_state_t state)
 #ifndef CONFIG_SUSPEND_SKIP_SYNC
 	trace_suspend_resume(TPS("sync_filesystems"), 0, true);
 	pr_info("Syncing filesystems ... ");
-	sys_sync();
+	ksys_sync();
 	pr_cont("done.\n");
 	trace_suspend_resume(TPS("sync_filesystems"), 0, false);
 #endif

kernel/power/user.c

@@ -224,7 +224,7 @@ static long snapshot_ioctl(struct file *filp, unsigned int cmd,
 			break;
 
 		printk("Syncing filesystems ... ");
-		sys_sync();
+		ksys_sync();
 		printk("done.\n");
 
 		error = freeze_processes();