Messages in this thread | | | Date | Wed, 15 Jul 2020 10:12:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 1/6] sched_clock: Expose struct clock_read_data |
| |
On Wed, Jul 15, 2020 at 07:56:50AM +0200, Ahmed S. Darwish wrote: > On Wed, Jul 15, 2020 at 10:05:07AM +0800, Leo Yan wrote: > > From: Peter Zijlstra <peterz@infradead.org> > > > ... > > > > Provide struct clock_read_data and two (seqcount) helpers so that > > architectures (arm64 in specific) can expose the numbers to userspace. > > > ... > > > > +struct clock_read_data *sched_clock_read_begin(unsigned int *seq) > > +{ > > + *seq = raw_read_seqcount(&cd.seq); > > + return cd.read_data + (*seq & 1); > > +} > > + > ... > > Hmm, this seqcount_t is actually a latch seqcount. I know the original > code also used raw_read_seqcount(), but while at it, let's use the > proper read API for seqcount_t latchers: raw_read_seqcount_latch(). > > raw_read_seqcount_latch() has no read memory barrier though, and a > suspicious claim that READ_ONCE() pairs with an smp_wmb() (??). But if > its implementation is wrong, let's fix it there instead.
It's supposed to be a dependent load, so READ_ONCE() is sufficient. Except, of course, the C standard has other ideas, so a compiler is allowed to wreck that, but they mostly don't :-)
| |