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

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Message-ID: <MWHPR21MB1593E4B1051F96DB6E715C0CD7C79@MWHPR21MB1593.namprd21.prod.outlook.com>
Date:   Thu, 26 Aug 2021 16:50:28 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Long Li <longli@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
        "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>
CC:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: RE: [PATCH] PCI: hv: Fix a bug on removing child devices on the bus

From: Long Li <longli@...rosoft.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.

Michael