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Date:   Wed, 14 Oct 2020 21:09:39 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
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        "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>,
        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: <20201015000939.GD6763@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>
 <20201009143209.GS5177@ziepe.ca>
 <CAPcyv4j54O8ac6WB3LEeNud2r11V26gA0PRKK9bhyEMF67AXtQ@mail.gmail.com>
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On Fri, Oct 09, 2020 at 11:28:54AM -0700, Dan Williams wrote:
> On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > 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.
> 
> It runs early because request_mem_region() can run before fs_initcall.
> Rather than add an unnecessary lock just arrange for the revoke to be
> skipped before the inode is initialized. The expectation is that any
> early resource reservations will block future userspace mapping
> attempts.

Actually, on this point a simple WRITE_ONCE/READ_ONCE pairing is OK,
Paul once explained that the pointer chase on the READ_ONCE side is
required to be like an acquire - this is why rcu_dereference is just
READ_ONCE

Jason