Re: [PATCH 3/4] dma-debug: Dynamically expand the dma_debug_entry pool

From: Robin Murphy
Date: Tue Dec 04 2018 - 11:32:08 EST


On 04/12/2018 14:29, Christoph Hellwig wrote:
+ 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 the freelist is non-empty we break out with the lock still held in order to actually allocate our entry - only if there are no free entries left do we drop the lock in order to handle the failure. This much is just the original logic shuffled around a bit (with the tweak that testing num_free_entries seemed justifiably simpler than the original list_empty() check).

+
+ 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?

Ah, right, we may be outside our own spinlock, but of course the whole DMA API call which got us here might be under someone else's and/or in a non-sleeping context - I'll fix that.

The number of retries is just to bound the loop due to its inherent raciness - since we drop the lock to create more entries, under pathological conditions by the time we get back in to grab one they could have all gone. 2 retries (well, strictly it's 1 try and 1 retry) was an entirely arbitrary choice just to accommodate that happening very occasionally by chance.

However, if the dynamic allocations need GFP_ATOMIC for external reasons anyway, then I don't need the lock-juggling that invites that race in the first place, and the whole loop disappears again. Neat!

Robin.