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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri, 9 Oct 2020 16:24:45 +0200
Message-ID: <CAKMK7uFpPP-Q0jC0vM7vYPEcg0m4NzTw+Ld=swdTF3BgMX5Qug@mail.gmail.com>
Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Jan Kara <jack@suse.cz>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        David Hildenbrand <david@redhat.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
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On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> 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.

Ah yes I checked this all, but forgot to correctly annotate the
iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
Add missing memory barriers for devmem_inode").

The reasons for the annotations is that iomem requests can happen
fairly early, way before fs_initcalls happen. That means revoke_iomem
needs to check for that and bail out if we race - nothing bad can
happen since userspace isn't running at this point anyway. And
apparently it needs to be a full acquire fence since we don't just
write a value, but need a barrier for the struct stuff.

Now iomem_get_mapping otoh can only be called after userspace is up &
running, so way after all the fs_initcalls are guaranteed to have
fininshed. Hence we don't really need anything there. But I expect the
kernel race checker thing to complain, plus that then gives me a good
spot to explain why we can't race and don't have to check for a NULL
iomem_inode.

I'll add that in v3.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch