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Date:   Fri, 11 Jun 2021 11:45:24 +0100
From:   Will Deacon <will@kernel.org>
To:     Ashish Mhetre <amhetre@nvidia.com>
Cc:     joro@8bytes.org, robin.murphy@arm.com, vdumpa@nvidia.com,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] iommu: Fix race condition during default domain
 allocation
Message-ID: <20210611104524.GD15274@willie-the-truck>
References: <1623298614-31755-1-git-send-email-amhetre@nvidia.com>
 <1623298614-31755-2-git-send-email-amhetre@nvidia.com>
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On Thu, Jun 10, 2021 at 09:46:53AM +0530, Ashish Mhetre wrote:
> Domain is getting created more than once during asynchronous multiple
> display heads(devices) probe. All the display heads share same SID and
> are expected to be in same domain. As iommu_alloc_default_domain() call
> is not protected, the group->default_domain and group->domain are ending
> up with different domains and leading to subsequent IOMMU faults.
> Fix this by protecting iommu_alloc_default_domain() call with group->mutex.

Can you provide some more information about exactly what the h/w
configuration is, and the callstack which exhibits the race, please?

> Signed-off-by: Ashish Mhetre <amhetre@nvidia.com>
> ---
>  drivers/iommu/iommu.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 808ab70..2700500 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -273,7 +273,9 @@ int iommu_probe_device(struct device *dev)
>  	 * support default domains, so the return value is not yet
>  	 * checked.
>  	 */
> +	mutex_lock(&group->mutex);
>  	iommu_alloc_default_domain(group, dev);
> +	mutex_unlock(&group->mutex);

It feels wrong to serialise this for everybody just to cater for systems
with aliasing SIDs between devices.

Will