linux-kernel - Re: [PATCH 2/5 V2] PCI: pciehp: check and wait port status out of DPC before handling DLLSC and PDC

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Message-ID: <20200929081800.GA15858@wunner.de>
Date:   Tue, 29 Sep 2020 10:18:00 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Sinan Kaya <okaya@...nel.org>
Cc:     Ethan Zhao <haifeng.zhao@...el.com>, bhelgaas@...gle.com,
        oohall@...il.com, ruscur@...sell.cc,
        andriy.shevchenko@...ux.intel.com, stuart.w.hayes@...il.com,
        mr.nuke.me@...il.com, mika.westerberg@...ux.intel.com,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, pei.p.jia@...el.com,
        ashok.raj@...ux.intel.com, sathyanarayanan.kuppuswamy@...el.com
Subject: Re: [PATCH 2/5 V2] PCI: pciehp: check and wait port status out of
 DPC before handling DLLSC and PDC

On Sun, Sep 27, 2020 at 11:27:46AM -0400, Sinan Kaya wrote:
> On 9/26/2020 11:28 PM, Ethan Zhao wrote:
> > --- a/drivers/pci/hotplug/pciehp_hpc.c
> > +++ b/drivers/pci/hotplug/pciehp_hpc.c
> > @@ -710,8 +710,10 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
> >  	down_read(&ctrl->reset_lock);
> >  	if (events & DISABLE_SLOT)
> >  		pciehp_handle_disable_request(ctrl);
> > -	else if (events & (PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_DLLSC))
> > +	else if (events & (PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_DLLSC)) {
> > +		pci_wait_port_outdpc(pdev);
> >  		pciehp_handle_presence_or_link_change(ctrl, events);
> > +	}
> >  	up_read(&ctrl->reset_lock);
> 
> This looks like a hack TBH.
> 
> Lukas, Keith;
> 
> What is your take on this?
> Why is device lock not protecting this situation?
> 
> Is there a lock missing in hotplug driver?

According to Ethan's commit message, there are two issues here:
One, that pciehp may remove a device even though DPC recovered the error,
and two, that a null pointer deref occurs.

The latter is most certainly not a locking issue but failure of DPC
to hold a reference on the pci_dev.

Thanks,

Lukas