linux-kernel - Re: [PATCH] memremap: Convert devmap static branch to {inc,dec}

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Message-Id: <4AF6A6E6-A098-448A-9CFD-86231D46032B@oracle.com>
Date:   Tue, 11 Aug 2020 05:59:14 -0600
From:   William Kucharski <william.kucharski@...cle.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memremap: Convert devmap static branch to {inc,dec}

Looks good to me.

Reviewed-by: William Kucharski <william.kucharski@...cle.com>

> On Aug 10, 2020, at 5:53 PM, ira.weiny@...el.com wrote:
> 
> From: Ira Weiny <ira.weiny@...el.com>
> 
> 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/
> 
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Vishal Verma <vishal.l.verma@...el.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> ---
> 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
> 
>