vfs: Fix sys_sync() and fsync_super() reliability (version 4)

So far, do_sync() called:
  sync_inodes(0);
  sync_supers();
  sync_filesystems(0);
  sync_filesystems(1);
  sync_inodes(1);

This ordering makes it kind of hard for filesystems as sync_inodes(0) need not
submit all the IO (for example it skips inodes with I_SYNC set) so e.g. forcing
transaction to disk in ->sync_fs() is not really enough. Therefore sys_sync has
not been completely reliable on some filesystems (ext3, ext4, reiserfs, ocfs2
and others are hit by this) when racing e.g. with background writeback. A
similar problem hits also other filesystems (e.g. ext2) because of
write_supers() being called before the sync_inodes(1).

Change the ordering of calls in do_sync() - this requires a new function
sync_blockdevs() to preserve the property that block devices are always synced
after write_super() / sync_fs() call.

The same issue is fixed in __fsync_super() function used on umount /
remount read-only.

[AV: build fixes]

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 3 insertions, 1 deletions

diff --git a/fs/sync.c b/fs/sync.c
index 7abc65fbf21d..631fd5aece78 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -13,6 +13,7 @@
 #include <linux/pagemap.h>
 #include <linux/quotaops.h>
 #include <linux/buffer_head.h>
+#include "internal.h"
 
 #define VALID_FLAGS (SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE| \
                         SYNC_FILE_RANGE_WAIT_AFTER)
@@ -26,10 +27,11 @@ static void do_sync(unsigned long wait)
         wakeup_pdflush(0);
         sync_inodes(0);         /* All mappings, inodes and their blockdevs */
         vfs_dq_sync(NULL);
+        sync_inodes(wait);      /* Mappings, inodes and blockdevs, again. */
         sync_supers();          /* Write the superblocks */
         sync_filesystems(0);    /* Start syncing the filesystems */
         sync_filesystems(wait); /* Waitingly sync the filesystems */
-        sync_inodes(wait);      /* Mappings, inodes and blockdevs, again. */
+        sync_blockdevs();
         if (!wait)
                 printk("Emergency Sync complete\n");
         if (unlikely(laptop_mode))