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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu, 15 Oct 2020 17:29:05 +0200
Message-ID: <CAKMK7uGeVzbe3=FR=a5MEfzDsrog6D4+Bkiaj8FrVeOLu3-9Mw@mail.gmail.com>
Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        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>,
        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 Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> 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

Hm so WRITE_ONCE doesn't have any barriers, and we'd need that for
updating the pointer. That would leave things rather inconsistent, so
I think I'll just leave it as-is for symmetry reasons. None of this
code matters for performance anyway, so micro-optimizing barriers
seems a bit silly.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch