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

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Message-ID: <20210826194113.yihk7ete4n4ej4gz@liuwe-devbox-debian-v2>
Date:   Thu, 26 Aug 2021 19:41:13 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     Long Li <longli@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
        "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>,
        "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

On Thu, Aug 26, 2021 at 04:50:28PM +0000, Michael Kelley wrote:
> 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.

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.

> 
> Michael