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Damien Le Moal 888cd7e40a ata: libata-scsi: avoid Non-NCQ command starvation 
commit 0ea84089db upstream.

When a non-NCQ command is issued while NCQ commands are being executed,
ata_scsi_qc_issue() indicates to the SCSI layer that the command issuing
should be deferred by returning SCSI_MLQUEUE_XXX_BUSY.  This command
deferring is correct and as mandated by the ACS specifications since
NCQ and non-NCQ commands cannot be mixed.

However, in the case of a host adapter using multiple submission queues,
when the target device is under a constant load of NCQ commands, there
are no guarantees that requeueing the non-NCQ command will be executed
later and it may be deferred again repeatedly as other submission queues
can constantly issue NCQ commands from different CPUs ahead of the
non-NCQ command. This can lead to very long delays for the execution of
non-NCQ commands, and even complete starvation for these commands in the
worst case scenario.

Since the block layer and the SCSI layer do not distinguish between
queueable (NCQ) and non queueable (non-NCQ) commands, libata-scsi SAT
implementation must ensure forward progress for non-NCQ commands in the
presence of NCQ command traffic. This is similar to what SAS HBAs with a
hardware/firmware based SAT implementation do.

Implement such forward progress guarantee by limiting requeueing of
non-NCQ commands from ata_scsi_qc_issue(): when a non-NCQ command is
received and NCQ commands are in-flight, do not force a requeue of the
non-NCQ command by returning SCSI_MLQUEUE_XXX_BUSY and instead return 0
to indicate that the command was accepted but hold on to the qc using
the new deferred_qc field of struct ata_port.

This deferred qc will be issued using the work item deferred_qc_work
running the function ata_scsi_deferred_qc_work() once all in-flight
commands complete, which is checked with the port qc_defer() callback
return value indicating that no further delay is necessary. This check
is done using the helper function ata_scsi_schedule_deferred_qc() which
is called from ata_scsi_qc_complete(). This thus excludes this mechanism
from all internal non-NCQ commands issued by ATA EH.

When a port deferred_qc is non NULL, that is, the port has a command
waiting for the device queue to drain, the issuing of all incoming
commands (both NCQ and non-NCQ) is deferred using the regular busy
mechanism. This simplifies the code and also avoids potential denial of
service problems if a user issues too many non-NCQ commands.

Finally, whenever ata EH is scheduled, regardless of the reason, a
deferred qc is always requeued so that it can be retried once EH
completes. This is done by calling the function
ata_scsi_requeue_deferred_qc() from ata_eh_set_pending(). This avoids
the need for any special processing for the deferred qc in case of NCQ
error, link or device reset, or device timeout.

Reported-by: Xingui Yang <yangxingui@huawei.com>
Reported-by: Igor Pylypiv <ipylypiv@google.com>
Fixes: bdb01301f3 ("scsi: Add host and host template flag 'host_tagset'")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Tested-by: Igor Pylypiv <ipylypiv@google.com>
Tested-by: Xingui Yang <yangxingui@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-02-26 14:59:46 -08:00
arch MIPS: Work around LLVM bug when gp is used as global register variable 2026-02-26 14:59:45 -08:00
block block,bfq: fix aux stat accumulation destination 2026-02-11 13:41:47 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec 2026-01-30 10:32:12 +01:00
Documentation ipv6: icmp: remove obsolete code in icmpv6_xrlim_allow() 2026-02-26 14:59:40 -08:00
drivers ata: libata-scsi: avoid Non-NCQ command starvation 2026-02-26 14:59:46 -08:00
fs ext4: use optimized mballoc scanning regardless of inode format 2026-02-26 14:59:45 -08:00
include ata: libata-scsi: avoid Non-NCQ command starvation 2026-02-26 14:59:46 -08:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring/cancel: de-unionize file and user_data in struct io_cancel_data 2026-02-26 14:59:36 -08:00
ipc ipc: don't audit capability check in ipc_permissions() 2026-02-26 14:59:19 -08:00
kernel bpf: Add a map/btf from a fd array more consistently 2026-02-26 14:59:38 -08:00
lib objpool: fix the overestimation of object pooling metadata size 2026-02-26 14:59:36 -08:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm/slab: fix false lockdep warning in __kfree_rcu_sheaf() 2026-02-26 14:59:14 -08:00
net ipv6: icmp: remove obsolete code in icmpv6_xrlim_allow() 2026-02-26 14:59:40 -08:00
rust rust: task: restrict Task::group_leader() to current 2026-02-26 14:59:18 -08:00
samples samples/ftrace: Adjust LoongArch register restore order in direct calls 2026-01-08 10:17:11 +01:00
scripts scripts: generate_rust_analyzer: Add compiler_builtins -> core dep 2026-02-06 16:57:42 +01:00
security apparmor: fix aa_label to return state from compount and component match 2026-02-26 14:59:42 -08:00
sound ASoC: rockchip: i2s-tdm: Use param rate if not provided by set_sysclk 2026-02-26 14:59:44 -08:00
tools selftests: tc_actions: don't dump 2MB of \0 to stdout 2026-02-26 14:59:40 -08:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: Don't clobber irqfd routing type when deassigning irqfd 2026-02-11 13:41:44 +01:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS soc: fixes for 6.18, part 4 2025-11-28 09:57:31 -08:00
Makefile kbuild: Add objtool to top-level clean target 2026-02-26 14:59:36 -08:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.