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())? > Also, a very long standing item on my TODO list is to see how much of it > we can unify across the various architectures, because there's a giant > amount of boiler plate involved with all this. Yeah, I'd be happy to help with that as a separate series. I already tripped over 5 or 6 page table walkers in arch/arm64/ alone :( Will
