linux-kernel - Re: [PATCH] iommu/dma: Relax locking in iommu_dma_prepare

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Message-ID: <2309c311-7378-385d-bf97-57965d36c18b@arm.com>
Date:   Fri, 18 Oct 2019 14:51:25 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     joro@...tes.org, iommu@...ts.linux-foundation.org, maz@...nel.org,
        julien.grall@....com, linux-kernel@...r.kernel.org,
        Qian Cai <cai@....pw>
Subject: Re: [PATCH] iommu/dma: Relax locking in iommu_dma_prepare_msi()

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.