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