Re: [PATCH v3 0/2] ata: fix deferred QC handling for port multipliers

[Tommy Kelly <linux@tkel.ly>]

On 5/8/26 12:50, Niklas Cassel wrote:
> Hello all,
> 
> Tommy Kelly reported a regression with PMP that use CBS:
> https://lore.kernel.org/linux-ide/ce09cc21-a8e9-4845-b205-35411e22fba9@tkel.ly/
> 
> It turns out that the ap->excl_link logic used for PMP that use CBS is
> incompatible with the deferred qc issuing via a workqueue.
> This is fixed in patch 1/2.
> 
> While looking at the code, it turns out that the deferred qc issuing via
> workqueue is misdesigned. It assumed that we can't mix NCQ and non-NCQ
> commands on the same port. The limitation is that you can not mix NCQ and
> non-NCQ commands on the same drive. However, with a PMP with FBS, you can
> issue (mixed NCQ and non-NCQ commands) to the different drives. Thus, move
> the saved deferred QC from struct ata_port to struct ata_link. This is
> fixed in patch 2/2.
> 
> 
> Changes since v1:
> -Avoid a local variable in sata_pmp_qc_defer_cmd_switch(), call
>  ata_std_qc_defer(qc) inside the switch() directly instead.
> -Fix a typo, such that the the code actually compiles... Sorry for that.
> 
> 
> Niklas Cassel (2):
>   ata: libata-scsi: do not use the deferred QC feature on PMPs with CBS
>   ata: libata-scsi: do not needlessly defer commands when using PMP with
>     FBS
> 
>  drivers/ata/libata-core.c | 16 ++++---
>  drivers/ata/libata-eh.c   |  8 ++--
>  drivers/ata/libata-pmp.c  | 26 +++++++++--
>  drivers/ata/libata-scsi.c | 95 ++++++++++++++++++++++++---------------
>  include/linux/libata.h    |  8 ++--
>  5 files changed, 101 insertions(+), 52 deletions(-)
> 

Hi Niklas,

Interestingly, I got my first warning with the v6.19.12 code.
I run fsck on the PMP drives on boot and during the "check_alloc_info" part, I got

> WARNING: drivers/ata/libata-scsi.c:1677 at ata_scsi_deferred_qc_work+0x84/0x98
> CPU: 0 UID: 0 PID: 157 Comm: kworker/0:1H Tainted: G           OE       6.19.12-200.fc43.aarch64 #1 PREEMPT(voluntary) 
> Hardware name: hardkernel Hardkernel ODROID-M1/Hardkernel ODROID-M1, BIOS 2026.07-rc1-dirty 07/01/2026
> Workqueue: events_highpri ata_scsi_deferred_qc_work
> pc : ata_scsi_deferred_qc_work+0x84/0x98
> lr : ata_scsi_deferred_qc_work+0x50/0x98
> Call trace:
>  ata_scsi_deferred_qc_work+0x84/0x98 (P)
>  process_one_work+0x17c/0x400
>  worker_thread+0x19c/0x328
>  kthread+0x10c/0x128
>  ret_from_fork+0x10/0x20
>
> WARNING: drivers/ata/libata-core.c:5143 at ata_qc_issue+0x34/0x420, CPU#0: kworker/0:1H/157
> CPU: 0 UID: 0 PID: 157 Comm: kworker/0:1H Tainted: G        W  OE       6.19.12-200.fc43.aarch64 #1 PREEMPT(voluntary) 
> Hardware name: hardkernel Hardkernel ODROID-M1/Hardkernel ODROID-M1, BIOS 2026.07-rc1-dirty 07/01/2026
> Workqueue: events_highpri ata_scsi_deferred_qc_work
> pc : ata_qc_issue+0x34/0x420
> lr : ata_scsi_deferred_qc_work+0x68/0x98
> Call trace:
>  ata_qc_issue+0x34/0x420 (P)
>  ata_scsi_deferred_qc_work+0x68/0x98
>  process_one_work+0x17c/0x400
>  worker_thread+0x19c/0x328
>  kthread+0x10c/0x128
>  ret_from_fork+0x10/0x20

Everything else on that boot seemed fine. No crashes or usability issues.

These are the lines that threw warnings:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/ata/libata-scsi.c?h=v6.19.12#n1677
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/ata/libata-core.c?h=v6.19.12#n5143

I haven't been able to reproduce it though, it only happened once.


And now for the latest code: To build, I ran

> git checkout -b fedora-7.0-libata kernel-ark/fedora-7.0 # linux v7.0.4
> git merge libata/for-7.1
> git cherry-pick --empty=drop v7.1-rc1..libata/for-7.1-fixes
> git cherry-pick --empty=drop v7.1-rc1..libata/for-next
> git cherry-pick 5918bf2a17f4689abfb94d341c5240088f8bd16e # FBS support
> b4 shazam 20260508195051.188207-4-cassel@kernel.org # v3 patch series

And the boot worked fine. No observable issues.
I haven't done any perf testing. If you'd instruct me how, I could run perf tests for you.
Maybe not torture tests though because I need the drives to survive. :)

Thank you,
- Tommy