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Date:   Wed, 22 Nov 2023 15:22:49 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Robin Murphy <robin.murphy@arm.com>, joro@8bytes.org,
        will@kernel.org, iommu@lists.linux.dev,
        linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org,
        rafael@kernel.org, lenb@kernel.org, lpieralisi@kernel.org,
        andre.draszik@linaro.org, quic_zhenhuah@quicinc.com
Subject: Re: [PATCH] iommu: Avoid more races around device probe
Message-ID: <20231122192249.GA399352@nvidia.com>
References: <16f433658661d7cadfea51e7c65da95826112a2b.1700071477.git.robin.murphy@arm.com>
 <ZVZL9kZuVsb9VPnM@nvidia.com>
 <2023112238-sierra-chewable-86fc@gregkh>
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On Wed, Nov 22, 2023 at 02:44:53PM +0000, Greg KH wrote:
> On Thu, Nov 16, 2023 at 01:05:58PM -0400, Jason Gunthorpe wrote:
> > On Wed, Nov 15, 2023 at 06:25:44PM +0000, Robin Murphy wrote:
> > > It turns out there are more subtle races beyond just the main part of
> > > __iommu_probe_device() itself running in parallel - the dev_iommu_free()
> > > on the way out of an unsuccessful probe can still manage to trip up
> > > concurrent accesses to a device's fwspec. Thus, extend the scope of
> > > iommu_probe_device_lock() to also serialise fwspec creation and initial
> > > retrieval.
> > > 
> > > Reported-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> > > Link: https://lore.kernel.org/linux-iommu/e2e20e1c-6450-4ac5-9804-b0000acdf7de@quicinc.com/
> > > Fixes: 01657bc14a39 ("iommu: Avoid races around device probe")
> > > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> > > ---
> > > 
> > > This is my idea of a viable fix, since it does not need a 700-line
> > > diffstat to make the code do what it was already *trying* to do anyway.
> > > This stuff should fundamentally not be hanging off driver probe in the
> > > first place, so I'd rather get on with removing the underlying
> > > brokenness than waste time and effort polishing it any further.
> > 
> > I'm fine with this as some hacky backport, but I don't want to see
> > this cross-layer leakage left in the next merge window.
> > 
> > ie we should still do my other series on top of and reverting this.
> > 
> > I've poked at moving parts of it under probe and I think we can do
> > substantial amounts in about two more series and a tidy a bunch of
> > other stuff too.
> 
> I agree, it's messy and acpi should not need this, BUT at the moment, I
> can't see any other way to resolve this simply.
> 
> So here's a begrudged ack:
> 
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> and hopefully the larger series should resolve this correctly?  Can that
> be rebased on top of this?

Yeah, I'm working on something more along the lines of Robin's desire
for a full reorganization.

The existing series has been tested by a few people now. We can decide
which order to put things in maybe next week if I get the new approach
done..

> Also, cc: stable on this for whomever applies it?

Also please update the commit message, the text from here does
describe the race:

https://lore.kernel.org/linux-iommu/11-v2-36a0088ecaa7+22c6e-iommu_fwspec_jgg@nvidia.com/

Jason