On 02/29/2016 04:28 PM, Tejun Heo wrote:
If cgroup writeback is in use, inodes can be scheduled for
asynchronous wb switching. Before 5ff8eaac1636 ("writeback: keep
superblock pinned during cgroup writeback association switches"), this
could race with umount leading to super_block being destroyed while
inodes are pinned for wb switching. 5ff8eaac1636 fixed it by bumping
s_active while wb switches are in flight; however, this allowed
in-flight wb switches to make umounts asynchronous when the userland
expected synchronosity - e.g. fsck immediately following umount may
fail because the device is still busy.
This patch removes the problematic super_block pinning and instead
makes generic_shutdown_super() flush in-flight wb switches. wb
switches are now executed on a dedicated isw_wq so that they can be
flushed and isw_nr_in_flight keeps track of the number of in-flight wb
switches so that flushing can be avoided in most cases.
v2: Move cgroup_writeback_umount() further below and add MS_ACTIVE
check in inode_switch_wbs() as Jan an Al suggested.
I queued this up for 4.5, but I'm feeling a bit uneasy about it. But
it's either that, or revert 5ff8eaac1636 and fix it for real in 4.6.
Jan/Tejun, what do you think?
--
Jens Axboe
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