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

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:

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From: Long Li <longli@microsoft.com> Sent: Wednesday, August 25, 2021
1:25 PM

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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.

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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:

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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.