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writeback, cgroup: fix use of the wrong bdi_writeback which mismatches the inode
When cgroup writeback is in use, there can be multiple wb's (bdi_writeback's) per bdi and an inode may switch among them dynamically. In a couple places, the wrong wb was used leading to performing operations on the wrong list under the wrong lock corrupting the io lists. * writeback_single_inode() was taking @wb parameter and used it to remove the inode from io lists if it becomes clean after writeback. The callers of this function were always passing in the root wb regardless of the actual wb that the inode was associated with, which could also change while writeback is in progress. Fix it by dropping the @wb parameter and using inode_to_wb_and_lock_list() to determine and lock the associated wb. * After writeback_sb_inodes() writes out an inode, it re-locks @wb and inode to remove it from or move it to the right io list. It assumes that the inode is still associated with @wb; however, the inode may have switched to another wb while writeback was in progress. Fix it by using inode_to_wb_and_lock_list() to determine and lock the associated wb after writeback is complete. As the function requires the original @wb->list_lock locked for the next iteration, in the unlikely case where the inode has changed association, switch the locks. Kudos to Tahsin for pinpointing these subtle breakages. Signed-off-by:Tejun Heo <tj@kernel.org> Fixes: d10c8095 ("writeback: implement foreign cgroup inode bdi_writeback switching") Link: http://lkml.kernel.org/g/CAAeU0aMYeM_39Y2+PaRvyB1nqAPYZSNngJ1eBRmrxn7gKAt2Mg@mail.gmail.com Reported-and-diagnosed-by: