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Date:   Tue, 29 Sep 2020 10:18:00 +0200
From:   Lukas Wunner <lukas@wunner.de>
To:     Sinan Kaya <okaya@kernel.org>
Cc:     Ethan Zhao <haifeng.zhao@intel.com>, bhelgaas@google.com,
        oohall@gmail.com, ruscur@russell.cc,
        andriy.shevchenko@linux.intel.com, stuart.w.hayes@gmail.com,
        mr.nuke.me@gmail.com, mika.westerberg@linux.intel.com,
        Keith Busch <keith.busch@intel.com>, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, pei.p.jia@intel.com,
        ashok.raj@linux.intel.com, sathyanarayanan.kuppuswamy@intel.com
Subject: Re: [PATCH 2/5 V2] PCI: pciehp: check and wait port status out of
 DPC before handling DLLSC and PDC
Message-ID: <20200929081800.GA15858@wunner.de>
References: <20200927032829.11321-1-haifeng.zhao@intel.com>
 <20200927032829.11321-3-haifeng.zhao@intel.com>
 <f2c9e3db-2027-f669-fcdd-fbc80888b934@kernel.org>
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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