On Thu, 2019-01-10 at 08:01 +1100, Dave Chinner wrote: > On Wed, Jan 09, 2019 at 03:53:29PM -0500, Qian Cai wrote: > > Easy to reproduce: > > > > 1. run LTP oom02 workload to let kswapd acquire this locking order: > > fs_reclaim -> sb_internal. > > > > # grep -i fs_reclaim -C 3 /proc/lockdep_chains | grep -C 5 sb_internal > > [00000000826b9172] &type->s_umount_key#27 > > [000000005fa8b2ac] sb_pagefaults > > [0000000033f1247e] sb_internal > > [000000009e9a9664] fs_reclaim > > > > 2. freeze XFS. > > # fsfreeze -f /home > > > > Dave mentioned that this is due to a lockdep limitation - "IOWs, this is > > a false positive, caused by the fact that xfs_trans_alloc() is called > > from both above and below memory reclaim as well as within /every level/ > > of freeze processing. Lockdep is unable to describe the staged flush > > logic in the freeze process that prevents deadlocks from occurring, and > > hence we will pretty much always see false positives in the freeze > > path....". Hence, just temporarily disable lockdep in that path. > > NACK. Turning off lockdep is not a solution, it just prevents > lockdep from finding and reporting real issues. > Well, it is a trade-off. It is turned on right after that path. All those false positives leave unfixed are also going to render lockdep less useful.
