[PATCH v2 2/2] mm/vmalloc: Use __this_cpu_try_cmpxchg() in preload_this_cpu_lock()

[Uros Bizjak]

Use __this_cpu_try_cmpxchg() instead of
__this_cpu_cmpxchg (*ptr, old, new) == old in
preload_this_cpu_lock().  x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg.

The generated code improves from:

    4bb6:	48 85 f6             	test   %rsi,%rsi
    4bb9:	0f 84 10 fa ff ff    	je     45cf <...>
    4bbf:	4c 89 e8             	mov    %r13,%rax
    4bc2:	65 48 0f b1 35 00 00 	cmpxchg %rsi,%gs:0x0(%rip)
    4bc9:	00 00
    4bcb:	48 85 c0             	test   %rax,%rax
    4bce:	0f 84 fb f9 ff ff    	je     45cf <...>

to:

    4bb6:	48 85 f6             	test   %rsi,%rsi
    4bb9:	0f 84 10 fa ff ff    	je     45cf <...>
    4bbf:	4c 89 e8             	mov    %r13,%rax
    4bc2:	65 48 0f b1 35 00 00 	cmpxchg %rsi,%gs:0x0(%rip)
    4bc9:	00 00
    4bcb:	0f 84 fe f9 ff ff    	je     45cf <...>

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: Dennis Zhou <dennis@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
---
v2: Show generated code improvement in the commit message.
---
 mm/vmalloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 5d3aa2dc88a8..4f34d935d648 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1816,7 +1816,7 @@ static void free_vmap_area(struct vmap_area *va)
 static inline void
 preload_this_cpu_lock(spinlock_t *lock, gfp_t gfp_mask, int node)
 {
-	struct vmap_area *va = NULL;
+	struct vmap_area *va = NULL, *tmp;
 
 	/*
 	 * Preload this CPU with one extra vmap_area object. It is used
@@ -1832,7 +1832,8 @@ preload_this_cpu_lock(spinlock_t *lock, gfp_t gfp_mask, int node)
 
 	spin_lock(lock);
 
-	if (va && __this_cpu_cmpxchg(ne_fit_preload_node, NULL, va))
+	tmp = NULL;
+	if (va && !__this_cpu_try_cmpxchg(ne_fit_preload_node, &tmp, va))
 		kmem_cache_free(vmap_area_cachep, va);
 }
 
-- 
2.42.0