On Wed, Oct 11, 2017 at 06:24:12PM +0200, Peter Zijlstra wrote: > On Wed, Oct 11, 2017 at 06:12:20PM +0200, Peter Zijlstra wrote: > > On Wed, Oct 11, 2017 at 08:59:48AM -0700, Paul E. McKenney wrote: > > > On Wed, Oct 11, 2017 at 04:17:25PM +0100, David Howells wrote: > > > > I will, however, quibble with the appropriateness of the name READ_ONCE()... > > > > I still think it's not sufficiently obvious that this is a barrier and the > > > > barrier is after. Maybe READ_AND_BARRIER()? > > > > > > Linus was unhappy with READ_ONCE_CTRL() to tag control dependencies, but > > > indicated that he might consider it if it helped code-analysis tools. > > > Adding Dmitry Vyukov for his thoughts on whether tagging READ_ONCE() > > > for dependencies would help. Me, I would suggest READ_ONCE_DEP(), but > > > let's figure out if the bikeshed needs to be painted before arguing over > > > the color. ;-) > > > > Count me one vote for the READ_ONCE() name. This is about dependent > > reads, which are nothing special on anything except Alpha. > > > > We want to remove the exception/specialness from the memory model; and > > therefore have to fix up all primitives that could possibly be used for > > these reads to unconditionally issue the barrier (on Alpha). The > > alternative is: rm -rf arch/alpha. > > > > Adding something like READ_ONCE_DEP() does not rid us of the idea that > > dependent reads are special and thus defeats the purpose, we might as > > well retain lockless_dereference(). > > > > Now; any user of dependent reads must use READ_ONCE() in any case, to > > avoid load tearing and reloads. So using READ_ONCE() for the dependent > > reads is not extra or additional (note we'll also have to add the > > barrier to all our relaxed and release atomics and anything else that > > implies READ_ONCE and doesn't already imply smp_mb() after). > > Add the per-cpu ops to that list, they imply READ_ONCE(). Consider for > example this example: > > > for_each_possible_cpu(cpu) > smp_store_release(per_cpu_ptr(&foo, cpu), obj); > > -vs- > > obj = this_cpu_read(foo); > if (obj->ponies) > fart_rainbow(obj); Interesting. Do we currently have any dependencies headed by this_cpu_read()? Thanx, Paul
