[PATCH] x86_64: Fix a race in the free_iommu path

We do this by removing a micro-optimization that tries to avoid grabbing
the iommu_bitmap_lock spinlock and using a bus-locked operation.

This still races with other simultaneous alloc_iommu or free_iommu(size >
1) which both use bus-unlocked operations.

The end result of this race is eventually ending up with an
iommu_gart_bitmap that has bits errornously set all over, making large
contiguous iommu space allocations fail with 'PCI-DMA: Out of IOMMU space'.
Signed-off-by: NMike Waychison <mikew@google.com>
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/pci-gart.c

@@ -112,10 +112,6 @@ static unsigned long alloc_iommu(int size)
 static void free_iommu(unsigned long offset, int size)
 { 
 	unsigned long flags;
-	if (size == 1) { 
-		clear_bit(offset, iommu_gart_bitmap); 
-		return;
-	}
 	spin_lock_irqsave(&iommu_bitmap_lock, flags);
 	__clear_bit_string(iommu_gart_bitmap, offset, size);
 	spin_unlock_irqrestore(&iommu_bitmap_lock, flags);