On Fri, Aug 31, 2018 at 04:22:41PM -0400, Dennis Zhou wrote:

Hi everyone,

This is a split of an earlier series I sent out [1] containing the first
3 patches with fixes from feedback. This series tackles the first
problem where blkcgs were not being destroyed.

There is a regression in blkcg destruction where references weren't
properly put causing blkcgs to never be destroyed. Previously, blkgs
were destroyed during offlining of the blkcg. This puts back the blkcg
reference a blkg holds allowing blkcg ref to reach zero. Then,
blkcg_css_free() is called as part of the final cleanup.

To address the problem, 0001 reverts the broken commit, 0002 delays
blkg destruction until writeback has finished, and 0003 closes the
window on a race condition between a css migration and dying, and
blkg association. This should fix the issue where blkg_get() was getting
called when a blkcg had already begun exiting. If a bio finds itself
here, it will just fall back to root. Oddly enough at one point,
blk-throttle was using policy data from and associating with potentially
different blkgs, thus how this was exposed.

For patches 1-3,

  Acked-by: Tejun Heo [off-list ref]

Thanks.

-- 
tejun