diff -urN ref/arch/alpha/kernel/pci_iommu.c iommu/arch/alpha/kernel/pci_iommu.c
--- ref/arch/alpha/kernel/pci_iommu.c	Tue May 22 17:39:07 2001
+++ iommu/arch/alpha/kernel/pci_iommu.c	Tue May 22 17:30:55 2001
@@ -238,7 +238,7 @@
 {
 	struct pci_controller *hose = pdev ? pdev->sysdata : pci_isa_hose;
 	struct pci_iommu_arena *arena;
-	long dma_ofs, npages;
+	long dma_ofs, npages, flags;
 
 	if (direction == PCI_DMA_NONE)
 		BUG();
@@ -269,6 +269,8 @@
 	}
 
 	npages = calc_npages((dma_addr & ~PAGE_MASK) + size);
+
+	spin_lock_irqsave(&arena->lock, flags);
 	iommu_arena_free(arena, dma_ofs, npages);
 
 
@@ -279,6 +281,7 @@
 	*/
 	if (dma_ofs >= arena->next_entry)
 		alpha_mv.mv_pci_tbi(hose, dma_addr, dma_addr + size - 1);
+	spin_unlock_irqrestore(&arena->lock, flags);
 
 	DBGA("pci_unmap_single: sg [%x,%lx] np %ld from %p\n",
 	     dma_addr, size, npages, __builtin_return_address(0));
@@ -566,6 +569,7 @@
 	struct scatterlist *end;
 	dma_addr_t max_dma;
 	dma_addr_t fbeg, fend;
+	long flags;
 
 	if (direction == PCI_DMA_NONE)
 		BUG();
@@ -580,6 +584,7 @@
 		arena = hose->sg_isa;
 
 	fbeg = -1, fend = 0;
+	spin_lock_irqsave(&arena->lock, flags);
 	for (end = sg + nents; sg < end; ++sg) {
 		unsigned long addr, size;
 		long npages, ofs;
@@ -619,6 +624,7 @@
 	*/
 	if ((fend - arena->dma_base) >> PAGE_SHIFT >= arena->next_entry)
 		alpha_mv.mv_pci_tbi(hose, fbeg, fend);
+	spin_unlock_irqrestore(&arena->lock, flags);
 
 	DBGA("pci_unmap_sg: %d entries\n", nents - (end - sg));
 }