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

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: linux-s390 <linux-s390@vger.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	"Jan Kara" <jack@suse.cz>, "Kees Cook" <keescook@chromium.org>,
	"KVM list" <kvm@vger.kernel.org>,
	"David Hildenbrand" <david@redhat.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"DRI Development" <dri-devel@lists.freedesktop.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
	"open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>
Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework
Date: Fri, 9 Oct 2020 11:32:09 -0300	[thread overview]
Message-ID: <20201009143209.GS5177@ziepe.ca> (raw)
In-Reply-To: <CAKMK7uFpPP-Q0jC0vM7vYPEcg0m4NzTw+Ld=swdTF3BgMX5Qug@mail.gmail.com>

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
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