[LU-4509] clio can be stuck in osc_extent_wait

• Type: Bug
• Resolution: Fixed
• Priority: Blocker
• Fix Version/s: Lustre 2.6.0, Lustre 2.5.2
• Labels: mn4
• Severity: 3
• Assignee: Zhenyu Xu
• Reporter: Liang Zhen (Inactive)

Description

Assume scenario like this:
1) thread-1 held an active extent
2) thread-2 called flush cache, and marked this extent as "urgent" and "sync_wait"
3) thread-3 wants to write to the same extent, osc_extent_find will get "conflict" because this extent is "sync_wait", so it starts to wait...
4) cl_writeback_work has been scheduled by thread-4 to write some other extents, it has sent RPCs but not returned yet.
5) thread-1 finished his work, and called osc_extent_release()> osc_io_unplug_async()>ptlrpcd_queue_work(), but found cl_writeback_work is still running, so it's ignored (-EBUSY)
6) thread-3 is stuck because nobody will wake him up.

This is an issue I hit while testing DAOS, but it should be addressed in common code.

Issue Links

• is duplicated by: LU-4790 CPU soft lockups (60+ seconds) in ptlrpcd threads running IOR resulting in eviction
• is related to: LU-4576 Seeing kernel panics in osc_extent_wait for 2.4.0 Lustre clients