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

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Date:   Wed, 14 Oct 2020 21:09:39 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Kees Cook <keescook@...omium.org>,
        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 11:28:54AM -0700, Dan Williams wrote:
> On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@...pe.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@...pe.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