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

[Dan Williams]

On Thu, May 21, 2020 at 4:41 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Wed, May 20, 2020 at 09:39:49PM -0700, Dan Williams wrote:
> > On Wed, May 20, 2020 at 9:37 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > > On Wed, May 20, 2020 at 7:26 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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?
> > >
> > > > > +
> > > > > +     /* 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?
> >
> > I'm pretty sure they are needed, because I need the prior writes to
> > initialize the inode to be fenced before the final write to publish
> > the inode. I don't think WRITE_ONCE() enforces that prior writes have
> > completed.
>
> Completed, no, but I think it does enforce that they're visible to other
> CPUs before this write is visible to other CPUs.
>
> I'll quote relevant bits from the document ...
>
>  (2) Data dependency barriers.
>
>      A data dependency barrier is a weaker form of read barrier.  In the case
>      where two loads are performed such that the second depends on the result
>      of the first (eg: the first load retrieves the address to which the second
>      load will be directed), a data dependency barrier would be required to
>      make sure that the target of the second load is updated after the address
>      obtained by the first load is accessed.
>
> [...]
> SMP BARRIER PAIRING
> -------------------
> [...]
>         CPU 1                 CPU 2
>         ===============       ===============================
>         a = 1;
>         <write barrier>
>         WRITE_ONCE(b, &a);    x = READ_ONCE(b);
>                               <data dependency barrier>
>                               y = *x;
>

Oh, I read those <* barrier> lines as a requirement not an implied
side effect of READ/WRITE_ONCE(). I can see that WRITE_ONCE() is
effectively a barrier() and READ_ONCE() includes
smp_read_barrier_depends(). I'll drop.

>
> > > >
> > > > > +     /*
> > > > > +      * 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.
>
> I don't think I explained myself well enough.
>
> When we initialise devmem_inode, does devmem_inode->i_mapping->host point
> to somewhere other than devmem_inode?
>
> I appreciate in this function, inode->i_mapping->host will point to inode.
> But we're now changing i_mapping to be devmem_inode's i_mapping.  Why
> do we need to change devmem_inode's i_mapping->host pointer?
>

Yeah, mistook your comment. The setting of ->host is indeed redundant.