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    SubjectRe: [PATCH] vfs: Fix sys_sync() and fsync_super() reliability
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    On Wed, 2009-04-22 at 17:56 +0200, Jan Kara wrote:
    > 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_blkdevs() 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.
    >
    > Signed-off-by: Jan Kara <jack@suse.cz>
    > ---
    > fs/super.c | 25 ++++++++++++++++++++++++-
    > fs/sync.c | 3 ++-
    > include/linux/fs.h | 2 ++
    > 3 files changed, 28 insertions(+), 2 deletions(-)
    >
    > diff --git a/fs/super.c b/fs/super.c
    > index 786fe7d..4f56333 100644
    > --- a/fs/super.c
    > +++ b/fs/super.c
    > @@ -267,6 +267,7 @@ void __fsync_super(struct super_block *sb)
    > {
    > sync_inodes_sb(sb, 0);
    > vfs_dq_sync(sb);
    > + sync_inodes_sb(sb, 1);
    > lock_super(sb);
    > if (sb->s_dirt && sb->s_op->write_super)
    > sb->s_op->write_super(sb);
    > @@ -274,7 +275,6 @@ void __fsync_super(struct super_block *sb)
    > if (sb->s_op->sync_fs)
    > sb->s_op->sync_fs(sb, 1);
    > sync_blockdev(sb->s_bdev);
    > - sync_inodes_sb(sb, 1);
    > }
    >
    > /*
    > @@ -502,6 +502,29 @@ restart:
    > mutex_unlock(&mutex);
    > }
    >
    > +/*
    > + * Sync all block devices underlying some superblock
    > + */
    > +void sync_blockdevs(void)
    > +{
    > + struct super_block *sb;
    > +
    > + spin_lock(&sb_lock);
    > +restart:
    > + list_for_each_entry(sb, &super_blocks, s_list) {
    > + sb->s_count++;
    > + spin_unlock(&sb_lock);
    > + down_read(&sb->s_umount);
    > + if (sb->s_root)
    > + sync_blockdev(sb->s_bdev);
    > + up_read(&sb->s_umount);

    What's the point of going to all this trouble of upping the sb->s_count,
    and grabbing sb->s_umount if there is no sb->s_bdev to sync in the first
    place?

    > + spin_lock(&sb_lock);
    > + if (__put_super_and_need_restart(sb))
    > + goto restart;
    > + }
    > + spin_unlock(&sb_lock);
    > +}
    > +
    > /**
    > * get_super - get the superblock of a device
    > * @bdev: device to get the superblock for
    > diff --git a/fs/sync.c b/fs/sync.c
    > index 7abc65f..fa14e42 100644
    > --- a/fs/sync.c
    > +++ b/fs/sync.c
    > @@ -26,10 +26,11 @@ 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))
    > diff --git a/include/linux/fs.h b/include/linux/fs.h
    > index 5bed436..4bad02e 100644
    > --- a/include/linux/fs.h
    > +++ b/include/linux/fs.h
    > @@ -1942,6 +1942,7 @@ extern void bdput(struct block_device *);
    > extern struct block_device *open_by_devnum(dev_t, fmode_t);
    > extern void invalidate_bdev(struct block_device *);
    > extern int sync_blockdev(struct block_device *bdev);
    > +extern void sync_blockdevs(void);
    > extern struct super_block *freeze_bdev(struct block_device *);
    > extern void emergency_thaw_all(void);
    > extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
    > @@ -1951,6 +1952,7 @@ extern int fsync_no_super(struct block_device *);
    > #else
    > static inline void bd_forget(struct inode *inode) {}
    > static inline int sync_blockdev(struct block_device *bdev) { return 0; }
    > +static inline void sync_blockdevs(void) { }
    > static inline void invalidate_bdev(struct block_device *bdev) {}
    >
    > static inline struct super_block *freeze_bdev(struct block_device *sb)
    > --
    > 1.6.0.2
    >
    > --
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