On Thu, Sep 28, 2017 at 08:43:54AM -0700, Paul E. McKenney wrote: > On Thu, Sep 28, 2017 at 09:45:35AM +0100, Will Deacon wrote: > > On Thu, Sep 28, 2017 at 10:38:01AM +0200, Peter Zijlstra wrote: > > > On Wed, Sep 27, 2017 at 04:49:28PM +0100, Will Deacon wrote: > > > > In many cases, page tables can be accessed concurrently by either another > > > > CPU (due to things like fast gup) or by the hardware page table walker > > > > itself, which may set access/dirty bits. In such cases, it is important > > > > to use READ_ONCE/WRITE_ONCE when accessing page table entries so that > > > > entries cannot be torn, merged or subject to apparent loss of coherence. > > > > > > In fact, we should use lockless_dereference() for many of them. Yes > > > Alpha is the only one that cares about the difference between that and > > > READ_ONCE() and they do have the extra barrier, but if we're going to do > > > this, we might as well do it 'right' :-) > > > > I know this sounds daft, but I think one of the big reasons why > > lockless_dereference() doesn't get an awful lot of use is because it's > > such a mouthful! Why don't we just move the smp_read_barrier_depends() > > into READ_ONCE? Would anybody actually care about the potential impact on > > Alpha (which, frankly, is treading on thin ice given the low adoption of > > lockless_dereference())? > > This is my cue to ask my usual question... ;-) > > Are people still running mainline kernels on Alpha? (Added Alpha folks.) Yes. I run two Alpha build daemons that build the unofficial debian-alpha port. Debian popcon reports nine machines running Alpha, which are likely to be running the 4.12.y kernel which is currently in debian-alpha, (and presumably soon to be 4.13.y which is now built on Alpha in experimental). Cheers Michael
