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Description of problem:
When VDO looks up a data-block hash value in the UDS index an attempt to deduplicate the data block, there’s a timeout interval applied after which VDO will stop waiting for UDS and will assume no duplicate block is present. The request data structure (embedded within our long-lived data I/O tracking structures) is marked “timed out” until UDS actually responds, at which point, since UDS is no longer mucking with that request structure, VDO is free to reuse it for another deduplication request.
Unfortunately, there is a race between the “process UDS response” code path and the “handle UDS timeouts” code path, such that if UDS responds right around the timeout point, both can attempt to use the same embedded list-node structure at the same time for processing, and one path can detect the list corruption that can result, logging a message mentioning list_del and usually crashing. The stack trace in the log may show timeoutIndexOperations or finishIndexOperation (or both, if both threads should detect corruption).
Version-Release number of selected component (if applicable):
6.2.6.14-84.el8
How reproducible:
Uncommon but has happened multiple times
Steps to Reproduce:
1. Present a heavy write load to VDO
2. Increase the system load or slow the VDO (UDS) I/O such that timeouts happen for a significant fraction of UDS requests. (For testing, adding variable-length delays along both code paths can trigger the problem fairly often.)
Actual results:
“list_del” bug (corruption) detection, crash
Expected results:
Deduplication requests either get answers or time out; VDO continues on.
Additional info:
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (kmod-kvdo bug fix and enhancement update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2022:7778