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c470c1c9f6
Each transaction hold makes multiple calls to _alloc_meta_low to see if the transaction should be committed to refill allocators before the caller's hold is acquired and they can dirty blocks in the transaction. _alloc_meta_low was using a spinlock to sample the allocator list_head blocks to determine if there was space available. The lock and unlock stores were creating significant cacheline contention. The _alloc_meta_low calls are higher frequency than allocations. We can use a seqlock to have exclusive writers and allow concurrent _alloc_meta_low readers who retry if a writer intervenes. Signed-off-by: Zach Brown <zab@versity.com>