Re: [PATCH v2 0/2] ATA port deferred qc fixes

[Niklas Cassel <cassel@kernel.org>]

On Thu, Apr 02, 2026 at 02:22:49AM -0700, Tommy Kelly wrote:
> On 2/20/26 14:14, Damien Le Moal wrote:
> > The first patch addresses a use-after-free issue when a deferred qc
> > times out. The second patch avoids a call to a potentially sleeping
> > function while a port spinlock is held.
> > 
> > Changes from v1:
> >  - Corrected typo in patch 1 message, improved comment in code and added
> >    a WARN_ON_ONCE() call to verify that a timed out qc is not active.
> >  - Fixed patch 2 to not call ata_scsi_requeue_deferred_qc() without the
> >    port lock held. This call is in fact removed: it is not needed as
> >    ata_scsi_requeue_deferred_qc() is called in EH, which is always run
> >    when removing a port.
> > 
> > Damien Le Moal (2):
> >   ata: libata-eh: correctly handle deferred qc timeouts
> >   ata: libata-core: fix cancellation of a port deferred qc work
> > 
> >  drivers/ata/libata-core.c |  8 +++-----
> >  drivers/ata/libata-eh.c   | 22 +++++++++++++++++++---
> >  2 files changed, 22 insertions(+), 8 deletions(-)
> > 
> 
> 
> Hello, this is my first message on the LKML so please forgive me for
> likely not following etiquette.
> 
> I have what might be a regression from somewhere in the recent series of
> NCQ / QC patches.
> 
> I have a peculiar setup that might reveal the source of the bug. I'm
> using a SATA Port Multiplier (PMP), powered by JMB575, which expands 1
> SATA port to 5 drives.
> The device/driver/subsystem tree looks like rk3568-dwc-ahci -> scsi -> sda.
> The platform does not (yet) support FIS-Based Switching (FBS), so it is
> using Command-Based Switching (CBS). FBS support may land in Linux 7.1.
> 
> > kernel: ahci-dwc fc800000.sata: flags: ncq sntf pm led clo only pmp fbs pio slum part ccc apst
> > kernel: ahci-dwc fc800000.sata: port 0 is not capable of FBS
> 
> When switching from kernel 6.18.13 to 6.19.7, drive access became
> extremely slow, causing lock timeouts at the filesystem level. I solved
> the problem by changing /sys/block/sd*/device/queue_depth from 32 to 1.
> 
> I also looked through the recent LPM patches, but the drives'
> performance didn't improve after changing LPM settings for the scsi host
> in sysfs.
> 
> Let me know if I need to provide more details or logs. I can see
> COMRESET a lot in each device's SMART logs but they aren't timestamped
> so I haven't proved that they are recent. Perhaps the slow drive
> switching is causing command starvation.

Hello Tommy,

Igor recently found a bug in the deferred QC feature:
https://lore.kernel.org/linux-ide/20260412153637.475606-1-ipylypiv@google.com/

His fix patch has been queued up on libata/for-next, so perhaps you could try
the libata/for-next branch:
https://git.kernel.org/pub/scm/linux/kernel/git/libata/linux.git/log/?h=for-next

And see if you can still reproduce the problem there.


Kind regards,
Niklas