Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region

[Matthew Wilcox]

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.