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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Date:   Mon, 28 Sep 2020 12:43:16 -0400
From:   Sinan Kaya <okaya@...nel.org>
To:     "Zhao, Haifeng" <haifeng.zhao@...el.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "oohall@...il.com" <oohall@...il.com>,
        "ruscur@...sell.cc" <ruscur@...sell.cc>,
        "lukas@...ner.de" <lukas@...ner.de>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "stuart.w.hayes@...il.com" <stuart.w.hayes@...il.com>,
        "mr.nuke.me@...il.com" <mr.nuke.me@...il.com>,
        "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>,
        Keith Busch <keith.busch@...el.com>
Cc:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jia, Pei P" <pei.p.jia@...el.com>,
        "ashok.raj@...ux.intel.com" <ashok.raj@...ux.intel.com>,
        "Kuppuswamy, Sathyanarayanan" <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 9/28/2020 7:10 AM, Sinan Kaya wrote:
> On 9/27/2020 10:01 PM, Zhao, Haifeng wrote:
>> Sinan,
>>    I explained the reason why locks don't protect this case in the patch description part. 
>> Write side and read side hold different semaphore and mutex.
>>
> I have been thinking about it some time but is there any reason why we
> have to handle all port AER/DPC/HP events in different threads?
> 
> Can we go to single threaded event loop for all port drivers events?
> 
> This will require some refactoring but it wlll eliminate the lock
> nightmares we are having.
> 
> This means no sleeping. All sleeps need to happen outside of the loop.
> 
> I wanted to see what you all are thinking about this.
> 
> It might become a performance problem if the system is
> continuously observing a hotplug/aer/dpc events.
> 
> I always think that these should be rare events.

If restructuring would be too costly, the preferred solution should be
to fix the locks in hotplug driver rather than throwing there a random
wait call.