linux-kernel - Re: [RFC 1/2] PCI: acpiphp: enable slot only if it hasn't been enabled already

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Date:   Wed, 13 Dec 2023 17:06:30 +0100
From:   Igor Mammedov <imammedo@...hat.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
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, 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 1/2] PCI: acpiphp: enable slot only if it hasn't been
 enabled already

On Wed, Dec 13, 2023 at 2:01 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Dec 13, 2023 at 1:36 AM Igor Mammedov <imammedo@...hat.com> wrote:
> >
> > When SCSI_SCAN_ASYNC is enabled (either via config or via cmd line),
> > adding device to bus and enabling it will kick in async host scan
> >
> >  scsi_scan_host+0x21/0x1f0
> >  virtscsi_probe+0x2dd/0x350
> >  ..
> >  driver_probe_device+0x19/0x80
> >  ...
> >  driver_probe_device+0x19/0x80
> >  pci_bus_add_device+0x53/0x80
> >  pci_bus_add_devices+0x2b/0x70
> >  ...
> >
> > which will schedule a job for async scan. That however breaks
> > if there are more than one SCSI host behind bridge, since
> > acpiphp_check_bridge() will walk over all slots and try to
> > enable each of them regardless of whether they were already
> > enabled.
> > As result the bridge might be reconfigured several times
> > and trigger following sequence:
> >
> >   [cpu 0] acpiphp_check_bridge()
> >   [cpu 0]   enable_slot(a)
> >   [cpu 0]     configure bridge
> >   [cpu 0]     pci_bus_add_devices() -> scsi_scan_host(a1)
> >   [cpu 0]   enable_slot(b)
> >   ...
> >   [cpu 1] do_scsi_scan_host(a1) <- async jib scheduled for slot a
> >   ...
> >   [cpu 0]     configure bridge <- temporaly disables bridge
> >
> > and cause do_scsi_scan_host() failure.
> > The same race affects SHPC (but it manages to avoid hitting the race due to
> > 1sec delay when enabling slot).
> > To cover case of single device hotplug (at a time) do not attempt to
> > enable slot that have already been enabled.
> >
> > Fixes: 40613da52b13 ("PCI: acpiphp: Reassign resources on bridge if necessary")
> > Reported-by: Dongli Zhang <dongli.zhang@...cle.com>
> > Reported-by: iona Ebner <f.ebner@...xmox.com>
> > Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> > ---
> >  drivers/pci/hotplug/acpiphp_glue.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/pci/hotplug/acpiphp_glue.c b/drivers/pci/hotplug/acpiphp_glue.c
> > index 601129772b2d..6b11609927d6 100644
> > --- a/drivers/pci/hotplug/acpiphp_glue.c
> > +++ b/drivers/pci/hotplug/acpiphp_glue.c
> > @@ -722,7 +722,9 @@ static void acpiphp_check_bridge(struct acpiphp_bridge *bridge)
> >                                         trim_stale_devices(dev);
> >
> >                         /* configure all functions */
> > -                       enable_slot(slot, true);
> > +                       if (slot->flags != SLOT_ENABLED) {
> > +                               enable_slot(slot, true);
> > +                       }
>
> Shouldn't this be following the acpiphp_enable_slot() pattern, that is
>
> if (!(slot->flags & SLOT_ENABLED))
>          enable_slot(slot, true);
>
> Also the braces are redundant.

I'll fix up on respin if Bjorn is fine with the approach in general.

Patches need respin anyways to fix botched up white spacing.

>
> >                 } else {
> >                         disable_slot(slot);
> >                 }
> > --
>