[patch 4/6] mm/vmalloc: Check free space in vmap_block lockless

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

vb_alloc() unconditionally locks a vmap_block on the free list to check the
free space.

This can be done locklessly because vmap_block::free never increases, it's
only decreased on allocations.

Check the free space lockless and only if that succeeds, recheck under the
lock.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 mm/vmalloc.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2166,6 +2166,9 @@ static void *vb_alloc(unsigned long size
 	list_for_each_entry_rcu(vb, &vbq->free, free_list) {
 		unsigned long pages_off;
 
+		if (READ_ONCE(vb->free) < (1UL << order))
+			continue;
+
 		spin_lock(&vb->lock);
 		if (vb->free < (1UL << order)) {
 			spin_unlock(&vb->lock);
@@ -2174,7 +2177,7 @@ static void *vb_alloc(unsigned long size
 
 		pages_off = VMAP_BBMAP_BITS - vb->free;
 		vaddr = vmap_block_vaddr(vb->va->va_start, pages_off);
-		vb->free -= 1UL << order;
+		WRITE_ONCE(vb->free, vb->free - (1UL << order));
 		bitmap_set(vb->used_map, pages_off, (1UL << order));
 		if (vb->free == 0) {
 			spin_lock(&vbq->lock);