Public bug reported:
Ubuntu 4.15.0-46.49-generic 4.15.18
It happens because the process is waiting for its request completion which
never happens.
The reason for the hung request is a race condition inside the block
layer.
Namely, there is a race condition with a long request.
Each request has a timer. When timer fires it sets REQ_ATOM_COMPLETE and
clears it after finishing.
The request completion checks REQ_ATOM_COMPLETE and if it is set the completion
returns doing nothing and never executes again, thinking that the request
doesn't need any attention anymore since it's actually completed.
Thus, if the request completion starts executing when the timer handler is in
progress it just returns seeing that the complete flag is set, then the timer
clears the complete flag and the request stays in the system forever executing
the timer handler again and again which just rearms itself.
This happens with the long-running requests only. By default, the request
timeout is 30 seconds so there should be a request which execution time > 30
seconds.
This is a rare case for local hardware storages but may appear more often when
the storage is accessed via a network.
The behavior described affects mainstream 4.13, 4.14, 4.15 kernels and
rh7-3.10.0-957.5.1.el7 kernel based systems.
Before 4.13 - the timer didn't rearm itself and just aborted the request. The
patch rearming the timer was introduced in 4.13: e72c9a2a67a6400c "scsi:
virtio_scsi: let host do exception handling"
After 4.15 the block layer switched to using MQ scheme in block layer
which isn't prone to this kind of races. In recent kernel >=5.0 there is
the only MQ scheme left and the legacy race-prone block layer code has
been removed.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: block layer virtio-scsi
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https://bugs.launchpad.net/bugs/1821738
Title:
A userspace process hangs in d-state forever in a virtual machine
environment with a virtio-scsi disk
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