[PATCH] memremap: Convert devmap static branch to {inc,dec}

[ira . weiny]

From: Ira Weiny <ira.weiny@xxxxxxxxx>

While reviewing Protection Key Supervisor support it was pointed out
that using a counter to track static branch enable was an anti-pattern
which was better solved using the provided static_branch_{inc,dec}
functions.[1]

Fix up devmap_managed_key to work the same way.  Also this should be
safer because there is a very small (very unlikely) race when multiple
callers try to enable at the same time.

[1] https://lore.kernel.org/lkml/20200714194031.GI5523@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
---
 mm/memremap.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/mm/memremap.c b/mm/memremap.c
index 03e38b7a38f1..9fb9ad500e78 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -40,12 +40,10 @@ EXPORT_SYMBOL_GPL(memremap_compat_align);
 #ifdef CONFIG_DEV_PAGEMAP_OPS
 DEFINE_STATIC_KEY_FALSE(devmap_managed_key);
 EXPORT_SYMBOL(devmap_managed_key);
-static atomic_t devmap_managed_enable;
 
 static void devmap_managed_enable_put(void)
 {
-	if (atomic_dec_and_test(&devmap_managed_enable))
-		static_branch_disable(&devmap_managed_key);
+	static_branch_dec(&devmap_managed_key);
 }
 
 static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
@@ -56,8 +54,7 @@ static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
 		return -EINVAL;
 	}
 
-	if (atomic_inc_return(&devmap_managed_enable) == 1)
-		static_branch_enable(&devmap_managed_key);
+	static_branch_inc(&devmap_managed_key);
 	return 0;
 }
 #else
-- 
2.28.0.rc0.12.gb6a658bd00c9