RE: [PATCH v2 1/2] PCI: hv: Fix the PCI HyperV probe failure path to release resource properly

[Wei Hu <weh@microsoft.com>]

Hi Lorenzo,

Thanks for your review. Please see my comments inline. 

> -----Original Message-----
> From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Sent: Tuesday, May 5, 2020 11:03 PM
> To: Wei Hu <weh@microsoft.com>
> Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> wei.liu@kernel.org; robh@kernel.org; bhelgaas@google.com; linux-
> hyperv@vger.kernel.org; linux-pci@vger.kernel.org; linux-
> kernel@vger.kernel.org; Dexuan Cui <decui@microsoft.com>; Michael Kelley
> <mikelley@microsoft.com>
> Subject: Re: [PATCH v2 1/2] PCI: hv: Fix the PCI HyperV probe failure path to
> release resource properly
> 
> On Fri, May 01, 2020 at 01:36:17PM +0800, Wei Hu wrote:
> > Some error cases in hv_pci_probe() were not handled. Fix these error
> > paths to release the resourses and clean up the state properly.
> 
> This patch does more than that. It adds a variable to store the number of slots
> actually allocated - I presume to free only allocated on slots on the exit path.
> 
> Two patches required I am afraid.

Well, adding this variable is needed to make the call of "(void) hv_pci_bus_exit(hdev, true)" 
at the end to work and clean up the PCI state in the failure path. Just adding this variable
would not make any changes. So I think it may be better to put them in single patch?

> 
> > Signed-off-by: Wei Hu <weh@microsoft.com>
> > ---
> >  drivers/pci/controller/pci-hyperv.c | 20 ++++++++++++++++----
> >  1 file changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/pci/controller/pci-hyperv.c
> > b/drivers/pci/controller/pci-hyperv.c
> > index e15022ff63e3..e6fac0187722 100644
> > --- a/drivers/pci/controller/pci-hyperv.c
> > +++ b/drivers/pci/controller/pci-hyperv.c
> > @@ -480,6 +480,9 @@ struct hv_pcibus_device {
> >
> >  	struct workqueue_struct *wq;
> >
> > +	/* Highest slot of child device with resources allocated */
> > +	int wslot_res_allocated;
> 
> I don't understand why you need an int rather than a u32.
> 
> Furthermore, I think a bitmap is more appropriate for what this variable is used
> for.

So it can use a negative value (-1 in this case) to indicated none of any resources
has been allocated. Currently value between 0-255 indicating some resources 
have been allocated. Of course I can use 0xffffffff to indicate that if it were u32. But
it wouldn't make much difference, would it?

It would take 32 bytes for total 256 child slots in bitmap, while it only takes 4 bytes 
using int. It is not in critical path so scanning from the location one by one is not a big
deal.

> 
> > +
> >  	/* hypercall arg, must not cross page boundary */
> >  	struct hv_retarget_device_interrupt retarget_msi_interrupt_params;
> >
> > @@ -2847,7 +2850,7 @@ static int hv_send_resources_allocated(struct
> hv_device *hdev)
> >  	struct hv_pci_dev *hpdev;
> >  	struct pci_packet *pkt;
> >  	size_t size_res;
> > -	u32 wslot;
> > +	int wslot;
> >  	int ret;
> >
> >  	size_res = (hbus->protocol_version < PCI_PROTOCOL_VERSION_1_2)
> @@
> > -2900,6 +2903,8 @@ static int hv_send_resources_allocated(struct hv_device
> *hdev)
> >  				comp_pkt.completion_status);
> >  			break;
> >  		}
> > +
> > +		hbus->wslot_res_allocated = wslot;
> >  	}
> >
> >  	kfree(pkt);
> > @@ -2918,10 +2923,10 @@ static int hv_send_resources_released(struct
> hv_device *hdev)
> >  	struct hv_pcibus_device *hbus = hv_get_drvdata(hdev);
> >  	struct pci_child_message pkt;
> >  	struct hv_pci_dev *hpdev;
> > -	u32 wslot;
> > +	int wslot;
> >  	int ret;
> >
> > -	for (wslot = 0; wslot < 256; wslot++) {
> > +	for (wslot = hbus->wslot_res_allocated; wslot >= 0; wslot--) {
> >  		hpdev = get_pcichild_wslot(hbus, wslot);
> >  		if (!hpdev)
> >  			continue;
> > @@ -2936,8 +2941,12 @@ static int hv_send_resources_released(struct
> hv_device *hdev)
> >  				       VM_PKT_DATA_INBAND, 0);
> >  		if (ret)
> >  			return ret;
> > +
> > +		hbus->wslot_res_allocated = wslot - 1;
> 
> Do you really need to set it at every loop iteration ?
> 
> Moreover, I think a bitmap is better suited for what you are doing, given that
> you may skip some loop indexes on !hpdev.
>
The value is set in the loop whenever a resource was successfully released. It could
happen that it failed in the next iteration so the last one which had succeeded would be
recorded in this variable. It is needed  at the end of loop when this 
iteration succeeds. 

Once again since it is not in the critical path, just using an signed integer is straightforward, 
less error prone and a bit easier to maintain than bitmap in my opinion. 😊
 
> >  	}
> >
> > +	hbus->wslot_res_allocated = -1;
> > +
> >  	return 0;
> >  }
> >
> > @@ -3037,6 +3046,7 @@ static int hv_pci_probe(struct hv_device *hdev,
> >  	if (!hbus)
> >  		return -ENOMEM;
> >  	hbus->state = hv_pcibus_init;
> > +	hbus->wslot_res_allocated = -1;
> >
> >  	/*
> >  	 * The PCI bus "domain" is what is called "segment" in ACPI and
> > other @@ -3136,7 +3146,7 @@ static int hv_pci_probe(struct hv_device
> > *hdev,
> >
> >  	ret = hv_pci_allocate_bridge_windows(hbus);
> >  	if (ret)
> > -		goto free_irq_domain;
> > +		goto exit_d0;
> >
> >  	ret = hv_send_resources_allocated(hdev);
> >  	if (ret)
> > @@ -3154,6 +3164,8 @@ static int hv_pci_probe(struct hv_device *hdev,
> >
> >  free_windows:
> >  	hv_pci_free_bridge_windows(hbus);
> > +exit_d0:
> > +	(void) hv_pci_bus_exit(hdev, true);
> 
> Remove the (void) cast.
> 

Some tools (maybe lint?) could generate error/warning messages assuming the code fails 
to check the return value without such cast. Leaving the cast in place just indicates that
the return value is deliberately ignored.  

Thanks, 
Wei