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Date:   Tue, 14 Jul 2020 10:40:57 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     ira.weiny@intel.com
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Fenghua Yu <fenghua.yu@intel.com>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kselftest@vger.kernel.org
Subject: Re: [RFC PATCH 11/15] memremap: Add zone device access protection
Message-ID: <20200714084057.GP10769@hirez.programming.kicks-ass.net>
References: <20200714070220.3500839-1-ira.weiny@intel.com>
 <20200714070220.3500839-12-ira.weiny@intel.com>
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On Tue, Jul 14, 2020 at 12:02:16AM -0700, ira.weiny@intel.com wrote:

> +static pgprot_t dev_protection_enable_get(struct dev_pagemap *pgmap, pgprot_t prot)
> +{
> +	if (pgmap->flags & PGMAP_PROT_ENABLED && dev_page_pkey != PKEY_INVALID) {
> +		pgprotval_t val = pgprot_val(prot);
> +
> +		mutex_lock(&dev_prot_enable_lock);
> +		dev_protection_enable++;
> +		/* Only enable the static branch 1 time */
> +		if (dev_protection_enable == 1)
> +			static_branch_enable(&dev_protection_static_key);
> +		mutex_unlock(&dev_prot_enable_lock);
> +
> +		prot = __pgprot(val | _PAGE_PKEY(dev_page_pkey));
> +	}
> +	return prot;
> +}
> +
> +static void dev_protection_enable_put(struct dev_pagemap *pgmap)
> +{
> +	if (pgmap->flags & PGMAP_PROT_ENABLED && dev_page_pkey != PKEY_INVALID) {
> +		mutex_lock(&dev_prot_enable_lock);
> +		dev_protection_enable--;
> +		if (dev_protection_enable == 0)
> +			static_branch_disable(&dev_protection_static_key);
> +		mutex_unlock(&dev_prot_enable_lock);
> +	}
> +}

That's an anti-pattern vs static_keys, I'm thinking you actually want
static_key_slow_{inc,dec}() instead of {enable,disable}().