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 15:32:58 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Alexandru Gagniuc <mr.nuke.me@...il.com>,
        linux-pci@...r.kernel.org, alex_gagniuc@...lteam.com,
        austin_bolen@...l.com, shyam_iyer@...l.com,
        linux-kernel@...r.kernel.org,
        Jonathan Derrick <jonathan.derrick@...el.com>,
        Lukas Wunner <lukas@...ner.de>,
        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 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.

Surprise hot remove is empirically more reliable the less we interact
with hardware and firmware. That shouldn't be necessary, but is just an
unfortunate reality.