linux-kernel - Re: [PATCH 0/5] s390/pci: automatic error recovery

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Message-ID: <e739c2919f97e277849a1bc1324a20df6a7d59eb.camel@linux.ibm.com>
Date:   Tue, 07 Sep 2021 10:45:41 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     "Oliver O'Halloran" <oohall@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Linas Vepstas <linasvepstas@...il.com>,
        Russell Currey <ruscur@...sell.cc>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390@...r.kernel.org,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>
Subject: Re: [PATCH 0/5] s390/pci: automatic error recovery

On Tue, 2021-09-07 at 12:04 +1000, Oliver O'Halloran wrote:
> On Mon, Sep 6, 2021 at 7:49 PM Niklas Schnelle <schnelle@...ux.ibm.com> wrote:
> > Patch 3 I already sent separately resulting in the discussion below but without
> > a final conclusion.
> > 
> > https://lore.kernel.org/lkml/20210720150145.640727-1-schnelle@linux.ibm.com/
> > 
> > I believe even though there were some doubts about the use of
> > pci_dev_is_added() by arch code the existing uses as well as the use in the
> > final patch of this series warrant this export.
> 
> The use of pci_dev_is_added() in arch/powerpc was because in the past
> pci_bus_add_device() could be called before pci_device_add(). That was
> fixed a while ago so It should be safe to remove those calls now.

Hmm, ok that confirms Bjorns suspicion and explains how it came to be.
I can certainly sent a patch for that. This would then leave only the
existing use in s390 which I added because of a dead lock prevention
and explained here:
https://lore.kernel.org/lkml/87d15d5eead35c9eaa667958d057cf4a81a8bf13.camel@linux.ibm.com/

Plus the need to use it in the recovery code of this series. I think in
the EEH code the need for a similar check is alleviated by the checks
in the beginning of
arch/powerpc/kernel/eeh_driver.c:eeh_handle_normal_event() especially
eeh_slot_presence_check() which checks presence via the hotplug slot.
I guess we could use our own state tracking in a similar way but felt
like pci_dev_is_added() is the more logical choice.

> 
> > Patch 4 "PCI: Export pci_dev_lock()" is basically an extension to commit
> > e3a9b1212b9d ("PCI: Export pci_dev_trylock() and pci_dev_unlock()") which
> > already exported pci_dev_trylock(). In the final patch we make use of
> > pci_dev_lock() to wait for any other exclusive uses of the pdev to be finished
> > before starting recovery.
> 
> Hmm, I noticed the EEH
> (arch/powerpc/kernel/eeh_driver.c:eeh_pe_report_edev())  and the
> generic PCIe error recovery code (see
> drivers/pci/pcie/err.c:report_error_detected()) only call
> device_lock() before entering the driver's error handling callbacks. I
> wonder if they should be using pci_dev_lock() instead. The only real
> difference is that pci_dev_lock() will also block user space from
> accessing the device's config space while error recovery is in
> progress which seems sensible enough.

I agree that sounds reasonable.