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Subject: Re: [PATCH 2/5 V2] PCI: pciehp: check and wait port status out of DPC
 before handling DLLSC and PDC
From:   Sinan Kaya <okaya@kernel.org>
To:     "Zhao, Haifeng" <haifeng.zhao@intel.com>,
        "bhelgaas@google.com" <bhelgaas@google.com>,
        "oohall@gmail.com" <oohall@gmail.com>,
        "ruscur@russell.cc" <ruscur@russell.cc>,
        "lukas@wunner.de" <lukas@wunner.de>,
        "andriy.shevchenko@linux.intel.com" 
        <andriy.shevchenko@linux.intel.com>,
        "stuart.w.hayes@gmail.com" <stuart.w.hayes@gmail.com>,
        "mr.nuke.me@gmail.com" <mr.nuke.me@gmail.com>,
        "mika.westerberg@linux.intel.com" <mika.westerberg@linux.intel.com>,
        Keith Busch <keith.busch@intel.com>
Cc:     "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Jia, Pei P" <pei.p.jia@intel.com>,
        "ashok.raj@linux.intel.com" <ashok.raj@linux.intel.com>,
        "Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@intel.com>
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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Message-ID: <16431a60-027e-eca9-36f4-74d348e88090@kernel.org>
Date:   Mon, 28 Sep 2020 12:43:16 -0400
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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.