mm/vmalloc: use __this_cpu_try_cmpxchg() in preload_this_cpu_lock()

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.

Link: https://lkml.kernel.org/r/20240528144345.5980-2-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d0cbdd7c1e5bc880cbc7d6b77b9919884a391e5f..4d9e1cbba8c28f6780447d19f0efd142a5f0c94f 100644 (file)
--- 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);
 }