linux-kernel - Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework

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Message-ID: <20201009123109.GO5177@ziepe.ca>
Date:   Fri, 9 Oct 2020 09:31:09 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
        linux-s390@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource
 framework

On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:

> +struct address_space *iomem_get_mapping(void)
> +{
> +	return iomem_inode->i_mapping;

This should pair an acquire with the release below

> +	/*
> +	 * Publish /dev/mem initialized.
> +	 * Pairs with smp_load_acquire() in revoke_iomem().
> +	 */
> +	smp_store_release(&iomem_inode, inode);

However, this seems abnormal, initcalls rarely do this kind of stuff
with global data..

The kernel crashes if this fs_initcall is raced with
iomem_get_mapping() due to the unconditional dereference, so I think
it can be safely switched to a simple assignment.

Jason