linux-kernel - Re: [PATCH 00/12] error handling and pciehp maintenance

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Date:   Tue, 6 Nov 2018 17:21:00 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Keith Busch <keith.busch@...el.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Sinan Kaya <okaya@...nel.org>,
        Thomas Tai <thomas.tai@...cle.com>, poza@...eaurora.org,
        Lukas Wunner <lukas@...ner.de>, Christoph Hellwig <hch@....de>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/12] error handling and pciehp maintenance

On Tue, Nov 06, 2018 at 09:47:52AM -0700, Keith Busch wrote:
> On Tue, Nov 06, 2018 at 04:34:08PM +0000, Lorenzo Pieralisi wrote:
> > The question is whether we really need to dynamically patch the kernel
> > with ftrace to achieve what that patch does.
> > 
> > Furthermore, it would also be good to report what bugs we are actually
> > fixing, from what you are writing falling back to the current method if
> > !DYNAMIC_FTRACE_WITH_REGS is broken in many ways and I would start with
> > fixing the current behaviour with something that does not depend on arch
> > features that may not even be implemented.
> 
> There are two problems with the current method:
> 
>   1. It may dereference pci_dev after it was freed
>   2. The pci_dev's children inherit its fake pci_bus's ops on
>      enumeration
> 
> Both result in kernel panic.

That's my point, current test module is not robust, I wanted to ask if
there is a way to fix it that does not depend on arch features, because
if there is a dependency that is not met we are still not fixing the
current code, using it as a fallback can still create issues.

> The dynamic kernel patch just seemed like a cool way to inject errors
> without messing with the driver's structures. But if there's a more
> elegant way to do it, I'm all for it.

If you have a simple reproducer for the bugs I am happy to help you test
it (I can also apply arm64 DYNAMIC_FTRACE_WITH_REGS patches and test that
new code path if that's the final direction we are taking).

Thanks,
Lorenzo