Re: [PATCH] iommu/dma: Relax locking in iommu_dma_prepare_msi()

[Christoph Hellwig]

On Wed, Oct 16, 2019 at 06:07:36PM +0100, Robin Murphy wrote:
> @@ -1180,7 +1179,7 @@ int iommu_dma_prepare_msi(struct msi_desc *desc, phys_addr_t msi_addr)
>  	struct iommu_domain *domain = iommu_get_domain_for_dev(dev);
>  	struct iommu_dma_cookie *cookie;
>  	struct iommu_dma_msi_page *msi_page;
> -	unsigned long flags;
> +	static DEFINE_MUTEX(msi_prepare_lock);

Just a style nitpick, but I find locks declared inside functions
really weird.  In addition to that locks not embedded into a structure
and not directly next to variables or data structures they protect
really need a comment explaining what they are trying to serialize.