From: Ira Weiny <ira.weiny@intel.com>
Date: Tue, 13 Oct 2020 16:52:33 -0700
Subject: mm/memremap.c: convert devmap static branch to {inc,dec}
Git-commit: 433e7d3177544c8cf0b6375abd310b0ef023fe9d
Patch-mainline: v5.10-rc1
References: jsc#SLE-16387

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@worktop.programming.kicks-ass.net/

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lkml.kernel.org/r/20200810235319.2796597-1-ira.weiny@intel.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/memremap.c |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- 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)
@@ -55,8 +53,7 @@ static int devmap_managed_enable_get(str
 		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