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Date:   Fri, 4 Nov 2022 18:36:15 -0700
From:   Brian Norris <briannorris@chromium.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     joro@8bytes.org, will@kernel.org, iommu@lists.linux.dev,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iommu: Avoid races around device probe
Message-ID: <Y2W+DxePm8WcUn46@google.com>
References: <1946ef9f774851732eed78760a78ec40dbc6d178.1667591503.git.robin.murphy@arm.com>
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On Fri, Nov 04, 2022 at 07:51:43PM +0000, Robin Murphy wrote:
> We currently have 3 different ways that __iommu_probe_device() may be
> called, but no real guarantee that multiple callers can't tread on each
> other, especially once asynchronous driver probe gets involved. It would
> likely have taken a fair bit of luck to hit this previously, but commit
> 57365a04c921 ("iommu: Move bus setup to IOMMU device registration") ups
> the odds since now it's not just omap-iommu that may trigger multiple
> bus_iommu_probe() calls in parallel if probing asynchronously.
> 
> Add a lock to ensure we can't try to double-probe a device, and also
> close some possible race windows to make sure we're truly robust against
> trying to double-initialise a group via two different member devices.
> 
> Reported-by: Brian Norris <briannorris@chromium.org>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/iommu/iommu.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)

If I've tested appropriately (there's always room for operator error),
this seems to resolve the problems I reported:

Tested-by: Brian Norris <briannorris@chromium.org>

I haven't reviewed closely enough to know how precisely this is a
regression (your description sounds like you think the bug existed some
time before that), but based on testing, this sounds like:

Fixes: 57365a04c921 ("iommu: Move bus setup to IOMMU device
registration")

But even if not, the report could probably use:

Link: https://lore.kernel.org/lkml/Y1CHh2oM5wyHs06J@google.com/

And most of all, thanks!

Brian