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From:   Ethan Zhao <xerces.zhao@gmail.com>
Date:   Tue, 29 Sep 2020 10:28:08 +0800
Message-ID: <CAKF3qh3nBS=bbusT_na54qo-yBdM1vc38Lx7JGm0Ck2sZ6qoCA@mail.gmail.com>
Subject: Re: [PATCH 2/5 V2] PCI: pciehp: check and wait port status out of DPC
 before handling DLLSC and PDC
To:     "Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@intel.com>
Cc:     Sinan Kaya <okaya@kernel.org>,
        "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>,
        "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>
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On Tue, Sep 29, 2020 at 12:45 AM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@intel.com> wrote:
>
>
> On 9/28/20 9:43 AM, Sinan Kaya wrote:
> > 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.
> Since the current race condition is detected between DPC and
> hotplug, I recommend synchronizing them.

The locks are the first place to root cause and try to fix. but not so easy to
refactor the remove-scan-semaphore and the bus-walk-mutex. too expensive
work. --- rework every piece of code that uses them.

Thanks,
Ethan

>
> --
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer
>