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

From: Daniel Vetter <daniel.vetter@ffwll.ch>
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>,
	"Jérôme 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
Date: Thu, 15 Oct 2020 09:52:45 +0200	[thread overview]
Message-ID: <CAKMK7uGu8yWRAGZX-dz-sXHsGufXKm003J5zuQr2AwNMG4+DUw@mail.gmail.com> (raw)
In-Reply-To: <20201015000939.GD6763@ziepe.ca>

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

Indeed this changed with the sm_read_barrier_depends() removal a year
ago. Before that READ_ONCE and rcu_dereference where not actually the
same. I guess I'll throw a patch on top to switch that over too.
-Daniel


--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch