From: Greg KH <gregkh@linuxfoundation.org>
To: Alex_Gagniuc@dellteam.com
Cc: keith.busch@intel.com, helgaas@kernel.org, mr.nuke.me@gmail.com,
linux-pci@vger.kernel.org, Austin.Bolen@dell.com,
Shyam.Iyer@dell.com, linux-kernel@vger.kernel.org,
jonathan.derrick@intel.com, lukas@wunner.de, ruscur@russell.cc,
sbobroff@linux.ibm.com, oohall@gmail.com,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is disconnected
Date: Thu, 8 Nov 2018 14:51:09 -0800 [thread overview]
Message-ID: <20181108225109.GA3023@kroah.com> (raw)
In-Reply-To: <20d68e586fff4dcca5616d5056f6fc21@ausx13mps321.AMER.DELL.COM>
On Thu, Nov 08, 2018 at 10:49:08PM +0000, Alex_Gagniuc@Dellteam.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
next prev parent reply other threads:[~2018-11-08 22:51 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-18 22:15 [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is disconnected Alexandru Gagniuc
2018-11-06 0:32 ` Alex G.
2018-11-07 17:04 ` Derrick, Jonathan
2018-11-07 23:42 ` Bjorn Helgaas
2018-11-08 20:09 ` Bjorn Helgaas
2018-11-08 21:49 ` Keith Busch
2018-11-08 22:01 ` Greg Kroah-Hartman
2018-11-08 22:32 ` Keith Busch
2018-11-08 22:42 ` Greg Kroah-Hartman
2018-11-08 22:49 ` Alex_Gagniuc
2018-11-08 22:51 ` Greg KH [this message]
2018-11-08 23:06 ` Alex_Gagniuc
2018-11-12 5:49 ` Oliver O'Halloran
2018-11-12 20:05 ` Alex_Gagniuc
2018-11-13 5:02 ` Bjorn Helgaas
2018-11-13 22:39 ` Alex_Gagniuc
2018-11-13 22:52 ` Keith Busch
2018-11-14 0:31 ` Alex_Gagniuc
2018-11-14 5:59 ` Bjorn Helgaas
2018-11-14 19:22 ` Alex_Gagniuc
2018-11-14 19:41 ` Derrick, Jonathan
2018-11-14 20:23 ` Keith Busch
2018-11-14 20:52 ` Alex_Gagniuc
2018-11-14 20:58 ` Keith Busch
2018-11-15 6:24 ` Bjorn Helgaas
2018-11-16 0:19 ` Alex_Gagniuc
2018-11-08 23:03 ` Keith Busch
2018-11-09 7:29 ` Lukas Wunner
2018-11-09 11:32 ` Greg Kroah-Hartman
2018-11-09 16:36 ` Keith Busch
2018-11-08 22:20 ` Alex_Gagniuc
2018-11-09 7:11 ` Lukas Wunner
2018-11-12 5:48 ` Oliver O'Halloran
2018-12-27 19:28 ` Alex_Gagniuc
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