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Date:   Wed, 20 May 2020 19:26:28 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     gregkh@...uxfoundation.org, Arnd Bergmann <arnd@...db.de>,
        Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Russell King <linux@....linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the
 region

On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote:
> +static struct inode *devmem_inode;
> +
> +#ifdef CONFIG_IO_STRICT_DEVMEM
> +void revoke_devmem(struct resource *res)
> +{
> +	struct inode *inode = READ_ONCE(devmem_inode);
> +
> +	/*
> +	 * Check that the initialization has completed. Losing the race
> +	 * is ok because it means drivers are claiming resources before
> +	 * the fs_initcall level of init and prevent /dev/mem from
> +	 * establishing mappings.
> +	 */
> +	smp_rmb();
> +	if (!inode)
> +		return;

But we don't need the smp_rmb() here, right?  READ_ONCE and WRITE_ONCE
are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance)
so the smp_rmb() is superfluous ...

> +	/*
> +	 * Use a unified address space to have a single point to manage
> +	 * revocations when drivers want to take over a /dev/mem mapped
> +	 * range.
> +	 */
> +	inode->i_mapping = devmem_inode->i_mapping;
> +	inode->i_mapping->host = devmem_inode;

umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode?

> +
> +	/* publish /dev/mem initialized */
> +	smp_wmb();
> +	WRITE_ONCE(devmem_inode, inode);

As above, unnecessary barrier, I think.

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