Re: [RFC 2/2] PCI: acpiphp: slowdown hotplug if hotplugging multiple devices at a time

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 13 Dec 2023 00:13:37 -0800
Dongli Zhang <dongli.zhang@oracle.com> wrote:

> Hi Igor,
> 
> 
> On 12/12/23 16:36, Igor Mammedov wrote:
> > previous commit ("PCI: acpiphp: enable slot only if it hasn't been enabled already"
> > introduced a workaround to avoid a race between SCSI_SCAN_ASYNC job and
> > bridge reconfiguration in case of single HBA hotplug.
> > However in virt environment it's possible to pause machine hotplug several
> > HBAs and let machine run. That can hit the same race when 2nd hotplugged  
> 
> Would you mind helping explain what does "pause machine hotplug several HBAs and
> let machine run" indicate?

qemu example would be:
{qemu) stop
(qemu) device_add device_add vhost-scsi-pci,wwpn=naa.5001405324af0985,id=vhost01,bus=bridge1,addr=8
(qemu) device_add vhost-scsi-pci,wwpn=naa.5001405324af0986,id=vhost02,bus=bridge1,addr=0
(qemu) cont

this way when machine continues to run acpiphp code will see 2 HBAs at once
and try to process one right after another. So [1/2] patch is not enough
to cover above case, and hence the same hack SHPC employs by adding delay.
However 2 separate hotplug events as in your reproducer should be covered
by the 1st patch.

> Thank you very much!
> 
> Dongli Zhang
> 
> > HBA will start re-configuring bridge.
> > Do the same thing as SHPC and throttle down hotplug of 2nd and up
> > devices within single hotplug event.
> > 
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> >  drivers/pci/hotplug/acpiphp_glue.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/pci/hotplug/acpiphp_glue.c b/drivers/pci/hotplug/acpiphp_glue.c
> > index 6b11609927d6..30bca2086b24 100644
> > --- a/drivers/pci/hotplug/acpiphp_glue.c
> > +++ b/drivers/pci/hotplug/acpiphp_glue.c
> > @@ -37,6 +37,7 @@
> >  #include <linux/mutex.h>
> >  #include <linux/slab.h>
> >  #include <linux/acpi.h>
> > +#include <linux/delay.h>
> >  
> >  #include "../pci.h"
> >  #include "acpiphp.h"
> > @@ -700,6 +701,7 @@ static void trim_stale_devices(struct pci_dev *dev)
> >  static void acpiphp_check_bridge(struct acpiphp_bridge *bridge)
> >  {
> >  	struct acpiphp_slot *slot;
> > +        int nr_hp_slots = 0;
> >  
> >  	/* Bail out if the bridge is going away. */
> >  	if (bridge->is_going_away)
> > @@ -723,6 +725,10 @@ static void acpiphp_check_bridge(struct acpiphp_bridge *bridge)
> >  
> >  			/* configure all functions */
> >  			if (slot->flags != SLOT_ENABLED) {
> > +				if (nr_hp_slots)
> > +					msleep(1000);
> > +
> > +                                ++nr_hp_slots;
> >  				enable_slot(slot, true);
> >  			}
> >  		} else {  
>