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

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Date:   Fri, 9 Nov 2018 03:32:57 -0800
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Alexandru Gagniuc <mr.nuke.me@...il.com>,
        linux-pci@...r.kernel.org, keith.busch@...el.com,
        alex_gagniuc@...lteam.com, austin_bolen@...l.com,
        shyam_iyer@...l.com, linux-kernel@...r.kernel.org,
        Jonathan Derrick <jonathan.derrick@...el.com>,
        Russell Currey <ruscur@...sell.cc>,
        Sam Bobroff <sbobroff@...ux.ibm.com>,
        Oliver O'Halloran <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 Fri, Nov 09, 2018 at 08:29:53AM +0100, Lukas Wunner 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
> > 
> > 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.
> 
> Hm, to be honest if that's your impression I think you must have missed a
> large portion of the discussion we've been having over the past 2 years.
> 
> Please consider reading this LWN article, particularly the "Surprise
> removal" section, to get up to speed:
> 
> https://lwn.net/Articles/767885/
> 
> You seem to be assuming that all we care about is the *return value* of
> an mmio read.  However a transaction to a surprise removed device has
> side effects beyond returning all ones, such as a Completion Timeout
> which, with thousands of transactions in flight, added up to many seconds
> to handle removal of an NVMe array and occasionally caused MCEs.

Again, I still claim this is broken hardware/firmware :)

> It is not an option to just blindly carry out device accesses even though
> it is known the device is gone, Completion Timeouts be damned.

I don't disagree with you at all, and your other email is great with
summarizing the issues here.

What I do object to is somehow relying on that function call as knowing
that the device really is present or not.  It's a good hint, yes, but
driver authors still have to be able to handle the bad data coming back
from when the call races with the device being removed.

> However there is more to it than just Completion Timeouts, this is all
> detailed in the LWN article.

And that's a great article and your work here is much appreciated.  I
think we are in violent agreement :)

thanks,

greg k-h