On Sat, Jan 12, 2019 at 12:20:07AM +0100, Andrea Parri wrote: > On Fri, Jan 11, 2019 at 09:41:28AM -0800, Daniel Jordan wrote: > > On Fri, Jan 11, 2019 at 12:59:19PM +0300, Dan Carpenter wrote: > > > diff --git a/mm/swapfile.c b/mm/swapfile.c > > > index f0edf7244256..21e92c757205 100644 > > > --- a/mm/swapfile.c > > > +++ b/mm/swapfile.c > > > @@ -1048,9 +1048,12 @@ swp_entry_t get_swap_page_of_type(int type) > > > struct swap_info_struct *si; > > > pgoff_t offset; > > > > > > + if (type >= nr_swapfiles) > > > + goto fail; > > > + > > > > As long as we're worrying about NULL, I think there should be an smp_rmb here > > to ensure swap_info[type] isn't NULL in case of an (admittedly unlikely) racing > > swapon that increments nr_swapfiles. See smp_wmb in alloc_swap_info and the > > matching smp_rmb's in the file. And READ_ONCE's on either side of the barrier > > per LKMM. > > > > I'm adding Andrea (randomly selected from the many LKMM folks to avoid spamming > > all) who can correct me if I'm wrong about any of this. > > This is to confirm that your analysis seems correct to me: the barriers > should guarantee that get_swap_page_of_type() will observe the store to > swap_info[type] performed by alloc_swap_info() (or a "co"-later store), > provided get_swap_page_of_type() observes the increment of nr_swapfiles > performed by the (same instance of) alloc_swap_info(). That's good to hear, thanks for looking into it. > One clarification about the READ_ONCE() matter: the LKMM cannot handle > plain or unmarked (shared memory) accesses in their generality at the > moment (patches providing support for these accesses are in the making, > but they will take some time); IAC, I'm confident to anticipate that, > for the particular pattern in question (aka, MP), marking the accesses > to nr_swapfiles will be "LKMM-sane" (one way to achieve this would be > to convert nr_swapfiles to an atomic_t type...). I guess you mean we could either use READ_ONCE or make nr_swapfiles atomic, they're different ways of achieving the same thing. > I take the liberty of adding other LKMM folks (so that they can blame > me for "the spam"! ;-) ): I've learnt from experience that four or more > eyes are better than two when it comes to discuss these matters... ;-) Ok, it's fine with me as long as they blame you :) > > > si = swap_info[type];
