Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Dan Williams]

On Tue, May 19, 2020 at 11:46 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, May 19, 2020 at 12:03:06AM -0700, Dan Williams wrote:
> > +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;
>
> Which wmb() is this pairing with?
>
> > +static int devmem_init_inode(void)
> > +{
> > +     static struct vfsmount *devmem_vfs_mount;
> > +     static int devmem_fs_cnt;
> > +     struct inode *inode;
> > +     int rc;
> > +
> > +     rc = simple_pin_fs(&devmem_fs_type, &devmem_vfs_mount, &devmem_fs_cnt);
> > +     if (rc < 0) {
> > +             pr_err("Cannot mount /dev/mem pseudo filesystem: %d\n", rc);
> > +             return rc;
> > +     }
> > +
> > +     inode = alloc_anon_inode(devmem_vfs_mount->mnt_sb);
> > +     if (IS_ERR(inode)) {
> > +             rc = PTR_ERR(inode);
> > +             pr_err("Cannot allocate inode for /dev/mem: %d\n", rc);
> > +             simple_release_fs(&devmem_vfs_mount, &devmem_fs_cnt);
> > +             return rc;
> > +     }
> > +
> > +     /* publish /dev/mem initialized */
> > +     WRITE_ONCE(devmem_inode, inode);
> > +     smp_wmb();
> > +
> > +     return 0;
>
> ... is that this one?  I don't see what it's guarding against.  Surely if
> it's needed to ensure that the writes to 'inode' have happened before
> the write of the inode pointer, the smp_wmb() needs to be before the
> WRITE_ONCE, not after it?

Whoops, yes. Thanks for the catch.