On Thu, Sep 28, 2017 at 04:49:54PM +0100, Will Deacon wrote: > 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.) > > > > As always, if anyone is, we must continue to support Alpha, but sounds > > like time to check again. > > I'll be honest and say that I haven't updated mine for a while, but I do > have a soft spot for those machines :( Let's see what the Alpha folks say. I myself have had a close relationship with Alpha for almost 20 years, but I suspect that in my case it is more a hard spot on my head rather than a soft spot in my heart. ;-) Thanx, Paul
