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;
> > >
> > > > + /*
> > > > + * 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?
