mm/vmalloc: check free space in vmap_block lockless

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.

Link: https://lkml.kernel.org/r/20230525124504.750481992@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -2168,6 +2168,9 @@ static void *vb_alloc(unsigned long size, gfp_t gfp_mask)
 	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);
@@ -2176,7 +2179,7 @@ static void *vb_alloc(unsigned long size, gfp_t gfp_mask)
 
 		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);