writeback: synchronize sync(2) against cgroup writeback membership switches

writeback: synchronize sync(2) against cgroup writeback membership switches

sync_inodes_sb() can race against cgwb (cgroup writeback) membership
switches and fail to writeback some inodes.  For example, if an inode
switches to another wb while sync_inodes_sb() is in progress, the new
wb might not be visible to bdi_split_work_to_wbs() at all or the inode
might jump from a wb which hasn't issued writebacks yet to one which
already has.

This patch adds backing_dev_info->wb_switch_rwsem to synchronize cgwb
switch path against sync_inodes_sb() so that sync_inodes_sb() is
guaranteed to see all the target wbs and inodes can't jump wbs to
escape syncing.

v2: Fixed misplaced rwsem init.  Spotted by Jiufei.

Signed-off-by: Tejun Heo <[email protected]>
Reported-by: Jiufei Xue <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Acked-by: Jan Kara <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>