There is a long existing report about btrfs hangs at unmount time, waiting for qgroup. Jeff has submitted a patch for that, but never merged. https://patchwork.kernel.org/patch/10376585/ After re-digging the case, although Jeff's fix can solve the problem, the racy cause doesn't look correct to me. After all, close_ctree() wait for qgroup rescan before destroying related work queues. Thus as long as the work is queued, we can finish the wait without problem. Further digging into the bug, it looks like the deadlock is possible, and Jeff is right about the wait-for-never-queued-work part. But the racy part doesn't look possible, thus it should only happen when something wrong happened. Now with a proper cause analyse, we can craft a much smaller thus better fix (anyway I'm the guy to backport, smaller is always better). Changelog: v2: - Change the subject It's not about race. I got confused by the initial patch. - Change the cause analyse No need for any race. Also add analyse for all qgroup_rescan_init() callers to ensure no missing fixes. BTW, qgroup_rescan_init() uses BTRFS_QGROUP_STATUS_FLAG_RESCAN flag to determine if there is a conflicting rescan, thus it's not affected by the timing change. - Split the spinlock cleanup into another patch Qu Wenruo (2): btrfs: qgroup: Ensure qgroup_rescan_running is only set when the worker is at least queued btrfs: qgroup: Remove the unnecesaary spin lock for qgroup_rescan_running|queued fs/btrfs/qgroup.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) -- 2.25.0
