On Sat, Dec 26, 2020 at 04:56:41AM -0500, Shijie Luo wrote:
> The root cause is that when offline/onlines disks, the filesystem can easily get into
> a error state and this makes it change to be read-only. Function freeze_super() will hold
> all sb_writers rwsems including rwsem of SB_FREEZE_WRITE when filesystem not read-only,
> but thaw_super_locked() cannot release these while the filesystem suddenly become read-only,
> because the logic will go to out.
>
> freeze_super
> hold sb_writers rwsems
> sb->s_writers.frozen = SB_FREEZE_COMPLETE
> thaw_super_locked
> sb_rdonly
> sb->s_writers.frozen = SB_UNFROZEN;
> goto out // not release rwsems
>
> And at this time, if we call mnt_want_write(), the process will be blocked.
>
> This patch fixes this problem, when filesystem is read-only, just not to set sb_writers.frozen
> be SB_FREEZE_COMPLETE in freeze_super() and then release all rwsems in thaw_super_locked.
I really don't like that - you end up with a case when freeze_super() returns 0 *and*
consumes the reference it had been give.
> if (sb_rdonly(sb)) {
> - /* Nothing to do really... */
> - sb->s_writers.frozen = SB_FREEZE_COMPLETE;
> - up_write(&sb->s_umount);
> + deactivate_locked_super(sb);
> return 0;
> }
