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From: Long Li <longli@microsoft.com>
To: Wei Liu <wei.liu@kernel.org>
Cc: "Michael Kelley" <mikelley@microsoft.com>,
	"longli@linuxonhyperv.com" <longli@linuxonhyperv.com>,
	"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"KY Srinivasan" <kys@microsoft.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Dexuan Cui" <decui@microsoft.com>,
	"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Dan Carpenter" <dan.carpenter@oracle.com>
Subject: RE: [PATCH] PCI: hv: Fix a bug on removing child devices on the bus
Date: Thu, 26 Aug 2021 20:20:52 +0000	[thread overview]
Message-ID: <BY5PR21MB15068C4370B39A1A376F79ACCEC79@BY5PR21MB1506.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20210826201503.ycckbcpu3f6flbb6@liuwe-devbox-debian-v2>

> 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

  reply	other threads:[~2021-08-26 20:21 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-24  7:20 longli
2021-08-24 11:02 ` Wei Liu
2021-08-24 17:28   ` Long Li
2021-08-25 19:11     ` Michael Kelley
2021-08-25 20:25       ` Long Li
2021-08-26 16:50         ` Michael Kelley
2021-08-26 19:41           ` Wei Liu
2021-08-26 20:09             ` Long Li
2021-08-26 20:15               ` Wei Liu
2021-08-26 20:20                 ` Long Li [this message]
2021-08-25 20:32       ` Rob Herring
2021-08-24 12:25 ` Bjorn Helgaas
2021-08-24 17:30   ` Long Li

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