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
> 
> On Thu, Aug 26, 2021 at 08:09:19PM +0000, Long Li wrote:
> > > Subject: Re: [PATCH] PCI: hv: Fix a bug on removing child devices on
> > > the bus
> > >
> > > On Thu, Aug 26, 2021 at 04:50:28PM +0000, Michael Kelley wrote:
> > > > From: Long Li <longli@microsoft.com> Sent: Wednesday, August 25,
> > > > 2021
> > > > 1:25 PM
> > > >
> > > > > >
> > > > > > 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.
> > > > >  */
> > > > >
> > > >
> > > > Got it.  Thanks (and to Rob Herring).   I read that comment but
> > > > with the wrong assumptions and didn't understand it correctly.
> > > >
> > > > Interestingly, pci-hyperv.c has another case of looping through
> > > > this list and removing items where the _safe version is not used.
> > > > See pci_devices_present_work() where the missing children are
> > > > moved to a list on the stack.
> > >
> > > That can be converted too, I think.
> > >
> > > The original code is not wrong per-se. It is just not as concise as
> > > using list_for_each_entry_safe.
> > >
> > > Wei.
> >
> > I assume we are talking about the following code in
> pci_devices_present_work():
> >
> >                 list_for_each_entry(hpdev, &hbus->children, list_entry) {
> >                         if (hpdev->reported_missing) {
> >                                 found = true;
> >                                 put_pcichild(hpdev);
> >                                 list_move_tail(&hpdev->list_entry, &removed);
> >                                 break;
> >                         }
> >                 }
> >
> > This code is correct as there is a "break" after a list entry is
> > removed from the list. So there is no need to use the _safe version.
> > This code can be converted to use the _safe version.
> 
> After this block there is another block like
> 
>   while (!list_empty(removed)) {
> 	...
>   	list_del(...)
> 
>   }
> 
> I assumed Michael was referring to that block. :-)
> 
> Wei.

This block is also correct. We don't have a bug here but there is a better way to code it.

Long