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

[Rafael J. Wysocki]

On Wed, Dec 13, 2023 at 1:36 AM Igor Mammedov <imammedo@xxxxxxxxxx> 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@xxxxxxxxxx>
> Reported-by: iona Ebner <f.ebner@xxxxxxxxxxx>
> Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
> ---
>  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.

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