On Mon, 27 Apr 2009 16:43:48 +0200
Jan Kara <jack@xxxxxxx> wrote:
> So far, do_sync() called:
> sync_inodes(0);
> sync_supers();
> sync_filesystems(0);
> sync_filesystems(1);
> sync_inodes(1);
The description has me all confused.
> 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.
Is not really enough for what?
sync_fs(wait==0) is not supposed to be reliable - it's an advice to the
fs that it should push as much "easy" writeback into the queue as
possible. We'll do the real sync later, with sync_fs(wait==1).
> 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.
No sync can ever be reliable in the presence of concurrent write
activity, unless we freeze userspace.
> 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.
So it's all a bit unclear (to me) what this patch is trying to fix?
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> fs/super.c | 27 ++++++++++++++++++++++++++-
> fs/sync.c | 3 ++-
> include/linux/fs.h | 2 ++
> 3 files changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index 786fe7d..4826540 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 +502,31 @@ 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) {
> + if (!sb->s_bdev)
> + continue;
> + 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);
> +}
The comment doesn't match the implementation. This function syncs all
blockdevs underlying _all_ superblocks.
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