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

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