writeback: keep cleanup_offline_cgwb()'s scan out of syncs

On a filesystem mounted with lazytime, sync() and syncfs() can return
without persisting timestamp updates of inodes attached to a dying
cgroup wb, so a crash after the sync loses them. File data and sizes are
not affected.

Since commit f6988c90671e ("writeback: bound cleanup_offline_cgwb()
rescans by rotating scanned inodes"), isw_prepare_wbs_switch() moves the
inodes it walks to the newest end of the list. When a pass fills its isw
before reaching an inode that got its lazytime update after a sync's
cutoff, the walked inodes land past it on b_dirty_time, and the sync's
queue_io(), which stops at the first inode dirtied after the cutoff,
misses them. The late inode has to be one without a pending lazytime
update; one already on b_dirty_time keeps its position.

The window is narrow. cleanup_offline_cgwbs_workfn() skips wbs with
anything on b_dirty, b_io or b_more_io, so the scan has to land between
wb_writeback() fixing dirtied_before and taking list_lock for its first
queue_io(), unless a multi-pass drain started before the wb got dirty
IO.

Take wb_switch_rwsem for read across the scan, as
process_inode_switch_wbs() does for the switches.

Fixes: f6988c90671e ("writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes")
Cc: stable@vger.kernel.org
Suggested-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/all/0bc5d58533f10b4841807be3025daa11@kernel.org/
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Liz Fong-Jones <lizf@honeycomb.io>
Link: https://patch.msgid.link/20261017040710.1174009-1-lizf@honeycomb.io
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions