linux-kernel - Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region

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Date:   Wed, 20 May 2020 21:37:57 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Russell King <linux@....linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region

On Wed, May 20, 2020 at 7:26 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote:
> > +static struct inode *devmem_inode;
> > +
> > +#ifdef CONFIG_IO_STRICT_DEVMEM
> > +void revoke_devmem(struct resource *res)
> > +{
> > +     struct inode *inode = READ_ONCE(devmem_inode);
> > +
> > +     /*
> > +      * Check that the initialization has completed. Losing the race
> > +      * is ok because it means drivers are claiming resources before
> > +      * the fs_initcall level of init and prevent /dev/mem from
> > +      * establishing mappings.
> > +      */
> > +     smp_rmb();
> > +     if (!inode)
> > +             return;
>
> But we don't need the smp_rmb() here, right?  READ_ONCE and WRITE_ONCE
> are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance)
> so the smp_rmb() is superfluous ...

Is it? I did not grok that from Documentation/memory-barriers.txt.
READ_ONCE and WRITE_ONCE are certainly ordered with respect to each
other in the same function, but I thought they still depend on
barriers for smp ordering?

>
> > +     /*
> > +      * Use a unified address space to have a single point to manage
> > +      * revocations when drivers want to take over a /dev/mem mapped
> > +      * range.
> > +      */
> > +     inode->i_mapping = devmem_inode->i_mapping;
> > +     inode->i_mapping->host = devmem_inode;
>
> umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode?

Not if inode is coming from:

     mknod ./newmem c 1 1

...that's the problem that a unified inode solves. You can mknod all
you want, but mapping and mapping->host will point to a common
instance.

>
> > +
> > +     /* publish /dev/mem initialized */
> > +     smp_wmb();
> > +     WRITE_ONCE(devmem_inode, inode);
>
> As above, unnecessary barrier, I think.

Well, if you're not sure, how sure should I be?