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    Subject[PATCH] vfs: Fix sys_sync() and fsync_super() reliability
    Date
    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);
    + 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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