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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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