linux-kernel - Re: [RFC 0/2] PCI: acpiphp: workaround race between hotplug and SCSI_SCAN

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Message-ID: <20231213181115.GA1050010@bhelgaas>
Date:   Wed, 13 Dec 2023 12:11:15 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Igor Mammedov <imammedo@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Dongli Zhang <dongli.zhang@...cle.com>,
        linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
        mst@...hat.com, rafael@...nel.org, lenb@...nel.org,
        bhelgaas@...gle.com, mika.westerberg@...ux.intel.com,
        boris.ostrovsky@...cle.com, joe.jin@...cle.com,
        stable@...r.kernel.org, Fiona Ebner <f.ebner@...xmox.com>,
        Thomas Lamprecht <t.lamprecht@...xmox.com>
Subject: Re: [RFC 0/2] PCI: acpiphp: workaround race between hotplug and
 SCSI_SCAN_ASYNC job

On Wed, Dec 13, 2023 at 01:36:12AM +0100, Igor Mammedov wrote:
> Hacks to mask a race between HBA scan job and bridge re-configuration(s)
> during hotplug. 
> 
> I don't like it a bit but it something that could be done quickly
> and solves problems that were reported.

I agree, I don't like it either.  Adding a 1s delay doesn't address
the real problem, and putting in a band-aid like this means the real
problem would likely never be addressed.

At this point the best option I see is to revert these:

  cc22522fd55e2 ("PCI: acpiphp: Use pci_assign_unassigned_bridge_resources() only for non-root bus")
  40613da52b13f ("PCI: acpiphp: Reassign resources on bridge if necessary")

I hate the fact that reverting them would mean the root bus hotplug
and ACPI bus check notifications would become issues again.

But keeping these commits even though they add a new different problem
that breaks things for somebody else seems worse to me.

Bjorn

> Other options to discuss/possibly more invasive:
>  1: make sure pci_assign_unassigned_bridge_resources() doesn't reconfigure
>     bridge if it's not necessary.
>  2. make SCSI_SCAN_ASYNC job wait till hotplug is finished for all slots on
>     the bridge or somehow restart the job if it fails
>  3. any other ideas?
> 
> 
> 1st reported: https://lore.kernel.org/r/9eb669c0-d8f2-431d-a700-6da13053ae54@proxmox.com
> 
> CC: Dongli Zhang <dongli.zhang@...cle.com>
> CC: linux-acpi@...r.kernel.org
> CC: linux-pci@...r.kernel.org
> CC: imammedo@...hat.com
> CC: mst@...hat.com
> CC: rafael@...nel.org
> CC: lenb@...nel.org
> CC: bhelgaas@...gle.com
> CC: mika.westerberg@...ux.intel.com
> CC: boris.ostrovsky@...cle.com
> CC: joe.jin@...cle.com
> CC: stable@...r.kernel.org
> CC: linux-kernel@...r.kernel.org
> CC: Fiona Ebner <f.ebner@...xmox.com>
> CC: Thomas Lamprecht <t.lamprecht@...xmox.com>
> 
> Igor Mammedov (2):
>   PCI: acpiphp: enable slot only if it hasn't been enabled already
>   PCI: acpiphp: slowdown hotplug if hotplugging multiple devices at a
>     time
> 
>  drivers/pci/hotplug/acpiphp_glue.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> -- 
> 2.39.3
>