[PATCH V3 0/3] Ensure ordered namespace registration during async scan

[Maurizio Lombardi <mlombard@redhat.com>]

The NVMe fully asynchronous namespace scanning introduced in
commit 4e893ca81170 ("nvme-core: scan namespaces asynchronously")
significantly improved discovery times. However, it also introduced
non-deterministic ordering for namespace registration.

While kernel device names (/dev/nvmeXnY) are not guaranteed to be stable
across reboots, this unpredictable ordering has caused considerable user
confusion and has been perceived as a regression, leading to multiple bug
reports.

This series introduces a solution to enforce strict sequential
registration based on NSID order, entirely preserving the performance
benefits of the asynchronous scan approach.

Instead of adding an NVMe-specific hack, this series abstracts the
serialization mechanism currently open-coded in the SCSI subsystem
(drivers/scsi/scsi_scan.c) into a generic library helper called the
completion chain (compl_chain).

By enforcing a strict First-In, First-Out (FIFO) completion order for
asynchronous tasks, we can ensure that namespaces are allocated and
registered sequentially without blocking the underlying parallel discovery
processes.

PATCH 3 Refactors the existing SCSI asynchronous scanning implementation
to use the new compl_chain helper, stripping out the custom, open-coded task
list and reducing code duplication.

Original code:

$ nvme list
Node                  Generic               Namespace
--------------------- --------------------- ----------
/dev/nvme0n1          /dev/ng0n1            0x2
/dev/nvme0n2          /dev/ng0n2            0x1
/dev/nvme0n3          /dev/ng0n3            0x5
/dev/nvme0n4          /dev/ng0n4            0x3
/dev/nvme0n5          /dev/ng0n5            0x4
[...]
/dev/nvme0n10         /dev/ng0n10           0xa
/dev/nvme0n11         /dev/ng0n11           0x8
/dev/nvme0n12         /dev/ng0n12           0x12
/dev/nvme0n13         /dev/ng0n13           0x17
/dev/nvme0n14         /dev/ng0n14           0xc
/dev/nvme0n15         /dev/ng0n15           0x11
/dev/nvme0n16         /dev/ng0n16           0x14
/dev/nvme0n17         /dev/ng0n17           0x13
/dev/nvme0n18         /dev/ng0n18           0xe
/dev/nvme0n19         /dev/ng0n19           0xf


With this patch:

$ nvme list
Node                  Generic               Namespace
--------------------- --------------------- ----------
/dev/nvme0n1          /dev/ng0n1            0x1
/dev/nvme0n2          /dev/ng0n2            0x2
/dev/nvme0n3          /dev/ng0n3            0x3
/dev/nvme0n4          /dev/ng0n4            0x4
/dev/nvme0n5          /dev/ng0n5            0x5
/dev/nvme0n6          /dev/ng0n6            0x6
[...]
/dev/nvme0n10         /dev/ng0n10           0xa
/dev/nvme0n11         /dev/ng0n11           0xb
/dev/nvme0n12         /dev/ng0n12           0xc
/dev/nvme0n13         /dev/ng0n13           0xd
/dev/nvme0n14         /dev/ng0n14           0xe
/dev/nvme0n15         /dev/ng0n15           0xf
/dev/nvme0n16         /dev/ng0n16           0x10
/dev/nvme0n17         /dev/ng0n17           0x11
/dev/nvme0n18         /dev/ng0n18           0x12
/dev/nvme0n19         /dev/ng0n19           0x13

V3: fixed some comments
    PATCH 3: declare scanning_hosts as static
    remove "extern" keyword from scsi_complete_async_scans()
    prototype declaration

V2: create the compl_chain helper that both SCSI and NVMe can share

Maurizio Lombardi (3):
  lib: Introduce completion chain helper
  nvme-core: register namespaces in order during async scan
  scsi: Convert async scanning to use the completion chain helper

 drivers/nvme/host/core.c    |  94 +++++++++++++++++-----------
 drivers/nvme/host/nvme.h    |   2 +
 drivers/scsi/scsi_priv.h    |   2 +-
 drivers/scsi/scsi_scan.c    |  68 +++------------------
 include/linux/compl_chain.h |  35 +++++++++++
 lib/Makefile                |   2 +-
 lib/compl_chain.c           | 118 ++++++++++++++++++++++++++++++++++++
 7 files changed, 225 insertions(+), 96 deletions(-)
 create mode 100644 include/linux/compl_chain.h
 create mode 100644 lib/compl_chain.c

-- 
2.53.0