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

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Message-ID: <20181108225109.GA3023@kroah.com>
Date:   Thu, 8 Nov 2018 14:51:09 -0800
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Alex_Gagniuc@...lteam.com
Cc:     keith.busch@...el.com, 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 Thu, Nov 08, 2018 at 10:49:08PM +0000, Alex_Gagniuc@...lteam.com wrote:
> 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.

If you have firmware that crashes the system when you try to read from a
PCI device that was hot-removed, that is broken firmware and needs to be
fixed.  The kernel can not work around that as again, you will never win
that race.

thanks,

greg k-h