On Fri, Sep 29, 2017 at 10:08:43AM +0100, Will Deacon wrote: > On Thu, Sep 28, 2017 at 05:58:30PM -0700, Paul E. McKenney wrote: > > On Fri, Sep 29, 2017 at 07:59:09AM +1300, Michael Cree 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.) > > > > > > 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). > > > > I salute your dedication to Alpha! ;-) > > Ok, but where does that leave us wrt my initial proposal of moving > smp_read_barrier_depends() into READ_ONCE and getting rid of > lockless_dereference? > > Michael (or anybody else running mainline on SMP Alpha) -- would you be > able to give the diff below a spin and see whether there's a measurable > performance impact? This will be a sensitive test. The smp_read_barrier_depends() can be removed from lockless_dereference(). Without this removal Alpha will get two memory barriers from rcu_dereference() and friends. Thanx, Paul > Cheers, > > Will > > --->8 > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index e95a2631e545..0ce21e25492a 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -340,6 +340,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s > __read_once_size(&(x), __u.__c, sizeof(x)); \ > else \ > __read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \ > + smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \ > __u.__val; \ > }) > #define READ_ONCE(x) __READ_ONCE(x, 1) >
