erofs: kill hooked chains to avoid loops on deduplicated compressed images
After heavily stressing EROFS with several images which include a
hand-crafted image of repeated patterns for more than 46 days, I found
two chains could be linked with each other almost simultaneously and
form a loop so that the entire loop won't be submitted. As a
consequence, the corresponding file pages will remain locked forever.
It can be _only_ observed on data-deduplicated compressed images.
For example, consider two chains with five pclusters in total:
Chain 1: 2->3->4->5 -- The tail pcluster is 5;
Chain 2: 5->1->2 -- The tail pcluster is 2.
Chain 2 could link to Chain 1 with pcluster 5; and Chain 1 could link
to Chain 2 at the same time with pcluster 2.
Since hooked chains are all linked locklessly now, I have no idea how
to simply avoid the race. Instead, let's avoid hooked chains completely
until I could work out a proper way to fix this and end users finally
tell us that it's needed to add it back.
Actually, this optimization can be found with multi-threaded workloads
(especially even more often on deduplicated compressed images), yet I'm
not sure about the overall system impacts of not having this compared
with implementation complexity.
Fixes: 267f2492c8f7 ("erofs: introduce multi-reference pclusters (fully-referenced)")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Link: https://lore.kernel.org/r/20230526201459.128169-4-hsiangkao@linux.alibaba.com
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