swiotlb: save io_tlb_used to local variable before leaving critical section

When swiotlb is full, the kernel would print io_tlb_used. However, the
result might be inaccurate at that time because we have left the critical
section protected by spinlock.

Therefore, we backup the io_tlb_used into local variable before leaving
critical section.

Fixes: 83ca2594 ("swiotlb: dump used and total slots when swiotlb buffer is full")
Suggested-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

kernel/dma/swiotlb.c

@@ -445,6 +445,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 	unsigned long mask;
 	unsigned long offset_slots;
 	unsigned long max_slots;
+	unsigned long tmp_io_tlb_used;
 
 	if (no_iotlb_memory)
 		panic("Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");
@@ -531,10 +532,12 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 	} while (index != wrap);
 
 not_found:
+	tmp_io_tlb_used = io_tlb_used;
+
 	spin_unlock_irqrestore(&io_tlb_lock, flags);
 	if (!(attrs & DMA_ATTR_NO_WARN) && printk_ratelimit())
-		dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes), total %lu, used %lu\n",
-			 size, io_tlb_nslabs, io_tlb_used);
+		dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes), total %lu (slots), used %lu (slots)\n",
+			 size, io_tlb_nslabs, tmp_io_tlb_used);
 	return DMA_MAPPING_ERROR;
 found:
 	io_tlb_used += nslots;