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

[Robin Murphy]

On 17/10/2019 17:24, Christoph Hellwig wrote:
> 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.

Hmm, the lock itself is merely a glorified comment, it's named for the 
operation it protects, its entire existence spans 15 consecutive lines, 
and 27% of those lines are dedicated to explaining that it's technically 
redundant. Is there *really* anything that isn't clear from the context?

Robin.