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

[Long Li <longli@microsoft.com>]

> Subject: RE: [PATCH] PCI: hv: Fix a bug on removing child devices on the bus
> 
> From: Long Li <longli@microsoft.com> 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@linuxonhyperv.com
> wrote:
> > > > From: Long Li <longli@microsoft.com>
> > > >
> > > > 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@microsoft.com>
> > > > Cc: Haiyang Zhang <haiyangz@microsoft.com>
> > > > Cc: Stephen Hemminger <sthemmin@microsoft.com>
> > > > Cc: Wei Liu <wei.liu@kernel.org>
> > > > Cc: Dexuan Cui <decui@microsoft.com>
> > > > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > > > Cc: Rob Herring <robh@kernel.org>
> > > > Cc: "Krzysztof Wilczyński" <kw@linux.com>
> > > > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > > > Cc: Michael Kelley <mikelley@microsoft.com>
> > > > Cc: Dan Carpenter <dan.carpenter@oracle.com>
> > > > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > > > Signed-off-by: Long Li <longli@microsoft.com>
> > > > ---
> > > >  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

I think we need list_for_each_entry_safe() because we delete the list elements while going through them:

Here is the comment on list_for_each_entry_safe():
/**
 * Loop through the list, keeping a backup pointer to the element. This
 * macro allows for the deletion of a list element while looping through the
 * list.
 *
 * See list_for_each_entry for more details.
 */

> 
> >
> > >
> > > >  			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
> > > >