Re: [PATCH] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

[Tejun Heo]

Hello,

Resending - earlier send dropped the Cc list. Sorry for the noise.

How rcu_barrier() got out of sync, as best I can reconstruct:

- ec084de929e4 ("fs/writeback.c: use rcu_barrier() to wait for inflight
  wb switches going into workqueue when umount", 2019) put the inc
  after call_rcu(); rcu_barrier() worked from then.

- 8826ee4fe750 ("writeback, cgroup: increment isw_nr_in_flight before
  grabbing an inode", 2021) moved the inc back ahead to cover the prep
  window, apparently reopening this gap.

- e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when
  switching inodes", 2025) replaced call_rcu() with llist_add() +
  queue_work(); rcu_barrier() looks like a no-op for this path since.

Could SRCU work instead? srcu_read_lock around the publish (atomic_inc
through wb_queue_isw), with cgroup_writeback_umount() keeping the
counter gate but swapping rcu_barrier() for synchronize_srcu():

  if (atomic_read(&isw_nr_in_flight)) {
          synchronize_srcu(&isw_srcu);
          flush_workqueue(isw_wq);
  }

Thoughts?

Thanks.

--
tejun