Hi Joel, On Wed, Sep 04, 2019 at 09:24:18AM -0400, Joel Fernandes wrote: > On Wed, Sep 04, 2019 at 01:30:38PM +0200, Peter Zijlstra wrote: > > On Wed, Sep 04, 2019 at 06:16:16AM -0400, Steven Rostedt wrote: > > > On Mon, 2 Sep 2019 11:16:23 +0200 > > > Peter Zijlstra <pet...@infradead.org> wrote: > > > > > > > in sched_dl_period_handler(). And do: > > > > > > > > + preempt_disable(); > > > > max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * > > > > NSEC_PER_USEC; > > > > min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * > > > > NSEC_PER_USEC; > > > > + preempt_enable(); > > > > > > Hmm, I'm curious. Doesn't the preempt_disable/enable() also add > > > compiler barriers which would remove the need for the READ_ONCE()s here? > > > > They do add compiler barriers; but they do not avoid the compiler > > tearing stuff up. > > Neither does WRITE_ONCE() on some possibly buggy but currently circulating > compilers :(
Hmm. The example above is using READ_ONCE, which is a different kettle of frogs. > As Will said in: > https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/ > > void bar(u64 *x) > { > *(volatile u64 *)x = 0xabcdef10abcdef10; > } > > gives: > > bar: > mov w1, 61200 > movk w1, 0xabcd, lsl 16 > str w1, [x0] > str w1, [x0, 4] > ret > > Speaking of which, Will, is there a plan to have compiler folks address this > tearing issue and are bugs filed somewhere? I believe aarch64 gcc is buggy, > and clang is better but is still buggy? Well, it depends on your point of view. Personally, I think tearing a volatile access (e.g. WRITE_ONCE) is buggy and it seems as though the GCC developers agree: https://gcc.gnu.org/ml/gcc-patches/2019-08/msg01500.html so it's likely this will be fixed for AArch64 GCC. I couldn't persuade clang to break the volatile case, so think we're good there too. For the non-volatile case, I don't actually consider it to be a bug, although I sympathise with the desire to avoid a retrospective tree-wide sweep adding random WRITE_ONCE invocations to stores that look like they might be concurrent. In other words, I think I'd suggest: * The use of WRITE_ONCE in new code (probably with a comment justifying it) * The introduction of WRITE_ONCE to existing code where it can be shown to be fixing a real bug (e.g. by demonstrating that a compiler actually gets it wrong) For the /vast/ majority of cases, the compiler will do the right thing even without WRITE_ONCE, simply because that's going to be the most performant choice as well. Will
