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

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Wed, May 06, 2020 at 05:36:46AM +0000, Wei Hu wrote:
> 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)" 

I don't understand why - it is not clear from the commit log and the
code, please explain it since it is not obvious.

> 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 is fine by me - I would not have written it this way but it does
not matter.

> 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.

I suggested a bitmap for correctness - a slot number may include slots
that as far as I can read the code failed get_pcichild_slot().

It is not clear what this patch is doing in this loop, that's certain.

> > 
> > > +
> > >  	/* 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. 

Ok understood.

> 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. 😊
>  

Less error prone, not sure, see above - it is your code so you choose
but please explain this change better, it is not obvious.

> > >  	}
> > >
> > > +	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.  

I understand that - the question is why it is OK to ignore it in this
specific case.

Maybe adding a wrapper around hv_pci_bus_exit() can help, I am fine with
it, just trying to help.

Lorenzo