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 :( 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? thanks, - Joel
