linux-kernel - Re: [PATCH 3/4] dma-debug: Dynamically expand the dma_debug

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Date:   Tue, 4 Dec 2018 15:29:38 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     hch@....de, m.szyprowski@...sung.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        cai@....us, salil.mehta@...wei.com, john.garry@...wei.com
Subject: Re: [PATCH 3/4] dma-debug: Dynamically expand the dma_debug_entry
 pool

> +	for (retry_count = 0; ; retry_count++) {
> +		spin_lock_irqsave(&free_entries_lock, flags);
> +
> +		if (num_free_entries > 0)
> +			break;
>  
>  		spin_unlock_irqrestore(&free_entries_lock, flags);

Taking a spinlock just to read a single integer value doesn't really
help anything.

> +
> +		if (retry_count < DMA_DEBUG_DYNAMIC_RETRIES &&
> +		    !prealloc_memory(DMA_DEBUG_DYNAMIC_ENTRIES))

Don't we need GFP_ATOMIC here?  Also why do we need the retries?