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Date:   Fri, 9 Oct 2020 11:32:09 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
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?SsOpcsO0bWU=?= Glisse <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>
Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource
 framework
Message-ID: <20201009143209.GS5177@ziepe.ca>
References: <20201009075934.3509076-1-daniel.vetter@ffwll.ch>
 <20201009075934.3509076-15-daniel.vetter@ffwll.ch>
 <20201009123109.GO5177@ziepe.ca>
 <CAKMK7uFpPP-Q0jC0vM7vYPEcg0m4NzTw+Ld=swdTF3BgMX5Qug@mail.gmail.com>
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On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote:
> 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").

Oh yikes, so revoke_iomem can run concurrently during early boot,
tricky.

> 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.

Yes, if that is what is happening it release/acquire is needed.

Jason