linux-kernel - Re: [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is disconnected

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Date:   Thu, 8 Nov 2018 22:49:08 +0000
From:   <Alex_Gagniuc@...lteam.com>
To:     <gregkh@...uxfoundation.org>, <keith.busch@...el.com>
Cc:     <helgaas@...nel.org>, <mr.nuke.me@...il.com>,
        <linux-pci@...r.kernel.org>, <Austin.Bolen@...l.com>,
        <Shyam.Iyer@...l.com>, <linux-kernel@...r.kernel.org>,
        <jonathan.derrick@...el.com>, <lukas@...ner.de>,
        <ruscur@...sell.cc>, <sbobroff@...ux.ibm.com>, <oohall@...il.com>,
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is
 disconnected

On 11/08/2018 04:43 PM, Greg Kroah-Hartman wrote:
> 
> [EXTERNAL EMAIL]
> Please report any suspicious attachments, links, or requests for sensitive information.
> 
> 
> On Thu, Nov 08, 2018 at 03:32:58PM -0700, Keith Busch wrote:
>> On Thu, Nov 08, 2018 at 02:01:17PM -0800, Greg Kroah-Hartman wrote:
>>> On Thu, Nov 08, 2018 at 02:09:17PM -0600, Bjorn Helgaas wrote:
>>>> I'm having second thoughts about this.  One thing I'm uncomfortable
>>>> with is that sprinkling pci_dev_is_disconnected() around feels ad hoc
>>>> instead of systematic, in the sense that I don't know how we convince
>>>> ourselves that this (and only this) is the correct place to put it.
>>>
>>> I think my stance always has been that this call is not good at all
>>> because once you call it you never really know if it is still true as
>>> the device could have been removed right afterward.
>>>
>>> So almost any code that relies on it is broken, there is no locking and
>>> it can and will race and you will loose.
>>
>> AIUI, we're not trying to create code to rely on this. This more about
>> reducing reliance on hardware. If the software misses the race once and
>> accesses disconnected device memory, that's usually not a big deal to
>> let hardware sort it out, but the point is not to push our luck.
> 
> Then why even care about this call at all?  If you need to really know
> if the read worked, you have to check the value.  If the value is FF
> then you have a huge hint that the hardware is now gone.  And you can
> rely on it being gone, you can never rely on making the call to the
> function to check if the hardware is there to be still valid any point
> in time after the call returns.

In the case that we're trying to fix, this code executing is a result of 
the device being gone, so we can guarantee race-free operation. I agree 
that there is a race, in the general case. As far as checking the result 
for all F's, that's not an option when firmware crashes the system as a 
result of the mmio read/write. It's never pretty when firmware gets 
involved.

Alex