On Mon, Jan 14, 2019 at 02:25:29PM -0800, Daniel Jordan wrote: > 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. Indeed: I was suggesting to mark the read _and the increment of nr_swapfiles, as I see you did in the patch you just submitted. Andrea > > swap_info[type] performed by alloc_swap_info() (or a "co"-later store), > > > 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];
