RE: [PATCH] PCI: hv: Fix a bug on removing child devices on the bus

[Michael Kelley]

From: Long Li <longli@xxxxxxxxxxxxx> Sent: Tuesday, August 24, 2021 10:28 AM
> 
> > Subject: Re: [PATCH] PCI: hv: Fix a bug on removing child devices on the bus
> >
> > On Tue, Aug 24, 2021 at 12:20:20AM -0700, longli@xxxxxxxxxxxxxxxxx wrote:
> > > From: Long Li <longli@xxxxxxxxxxxxx>
> > >
> > > In hv_pci_bus_exit, the code is holding a spinlock while calling
> > > pci_destroy_slot(), which takes a mutex.
> > >
> > > This is not safe for spinlock. Fix this by moving the children to be
> > > deleted to a list on the stack, and removing them after spinlock is
> > > released.
> > >
> > > Fixes: 94d22763207a ("PCI: hv: Fix a race condition when removing the
> > > device")
> > >
> > > Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
> > > Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> > > Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
> > > Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> > > Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
> > > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
> > > Cc: Rob Herring <robh@xxxxxxxxxx>
> > > Cc: "Krzysztof Wilczyński" <kw@xxxxxxxxx>
> > > Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > > Cc: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> > > Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > > Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
> > > ---
> > >  drivers/pci/controller/pci-hyperv.c | 15 ++++++++++++---
> > >  1 file changed, 12 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/pci/controller/pci-hyperv.c
> > > b/drivers/pci/controller/pci-hyperv.c
> > > index a53bd8728d0d..d4f3cce18957 100644
> > > --- a/drivers/pci/controller/pci-hyperv.c
> > > +++ b/drivers/pci/controller/pci-hyperv.c
> > > @@ -3220,6 +3220,7 @@ static int hv_pci_bus_exit(struct hv_device *hdev,
> > bool keep_devs)
> > >  	struct hv_pci_dev *hpdev, *tmp;
> > >  	unsigned long flags;
> > >  	int ret;
> > > +	struct list_head removed;
> >
> > This can be moved to where it is needed -- the if(!keep_dev) branch -- to limit its
> > scope.
> >
> > >
> > >  	/*
> > >  	 * After the host sends the RESCIND_CHANNEL message, it doesn't @@
> > > -3229,9 +3230,18 @@ static int hv_pci_bus_exit(struct hv_device *hdev, bool
> > keep_devs)
> > >  		return 0;
> > >
> > >  	if (!keep_devs) {
> > > -		/* Delete any children which might still exist. */
> > > +		INIT_LIST_HEAD(&removed);
> > > +
> > > +		/* Move all present children to the list on stack */
> > >  		spin_lock_irqsave(&hbus->device_list_lock, flags);
> > > -		list_for_each_entry_safe(hpdev, tmp, &hbus->children,
> > list_entry) {
> > > +		list_for_each_entry_safe(hpdev, tmp, &hbus->children,
> > list_entry)
> > > +			list_move_tail(&hpdev->list_entry, &removed);
> > > +		spin_unlock_irqrestore(&hbus->device_list_lock, flags);
> > > +
> > > +		/* Remove all children in the list */
> > > +		while (!list_empty(&removed)) {
> > > +			hpdev = list_first_entry(&removed, struct hv_pci_dev,
> > > +						 list_entry);
> >
> > list_for_each_entry_safe can also be used here, right?
> >
> > Wei.
> 
> I will address your comments.
> 
> Long

I thought list_for_each_entry_safe() is for use when list manipulation
is *not* protected by a lock and you want to safely walk the list
even if an entry gets removed.  If the list is protected by a lock or
not subject to contention (as is the case here), then
list_for_each_entry() is the simpler implementation.  The original
implementation didn't need to use the _safe version because of
the spin lock.

Or do I have it backwards?  

Michael

> 
> >
> > >  			list_del(&hpdev->list_entry);
> > >  			if (hpdev->pci_slot)
> > >  				pci_destroy_slot(hpdev->pci_slot);
> > > @@ -3239,7 +3249,6 @@ static int hv_pci_bus_exit(struct hv_device *hdev,
> > bool keep_devs)
> > >  			put_pcichild(hpdev);
> > >  			put_pcichild(hpdev);
> > >  		}
> > > -		spin_unlock_irqrestore(&hbus->device_list_lock, flags);
> > >  	}
> > >
> > >  	ret = hv_send_resources_released(hdev);
> > > --
> > > 2.25.1
> > >