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@xxxxxxxxxxxxx> 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.
