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:   Tue, 29 Sep 2020 10:50:03 +0800
From:   Ethan Zhao <xerces.zhao@...il.com>
To:     Sinan Kaya <okaya@...nel.org>
Cc:     "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>,
        "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 Tue, Sep 29, 2020 at 12:44 AM Sinan Kaya <okaya@...nel.org> 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.

  My first though is to unify the pci_bus_sem & pci_rescan_remove_lock
to one sleepable lock, but verifying every
locking scenario to sort out dead lock warning, it is horrible job. I
gave up and then played the device status waiting trick
to workaround it.

    index 03d37128a24f..477d4c499f87 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -3223,17 +3223,19 @@ EXPORT_SYMBOL_GPL(pci_rescan_bus);
  * pci_rescan_bus(), pci_rescan_bus_bridge_resize() and PCI device removal
  * routines should always be executed under this mutex.
  */
-static DEFINE_MUTEX(pci_rescan_remove_lock);
+/* static DEFINE_MUTEX(pci_rescan_remove_lock); */

 void pci_lock_rescan_remove(void)
 {
- mutex_lock(&pci_rescan_remove_lock);
+ /*mutex_lock(&pci_rescan_remove_lock); */
+ down_write(&pci_bus_sem);
 }
 EXPORT_SYMBOL_GPL(pci_lock_rescan_remove);

 void pci_unlock_rescan_remove(void)
 {
- mutex_unlock(&pci_rescan_remove_lock);
+ /*mutex_unlock(&pci_rescan_remove_lock); */
+ up_write(&pci_bus_sem);
 }
 EXPORT_SYMBOL_GPL(pci_unlock_rescan_remove);

Thanks，
Ethan