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SubjectRe: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount_latch()

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Apr 13, 2015 at 09:17:50PM +0200, Peter Zijlstra wrote:
> > On Mon, Apr 13, 2015 at 06:32:02PM +0200, Ingo Molnar wrote:
> > >
> > > Btw., I realize this is just a sample, but couldn't this be written
> > > more optimally as:
> > >
> > > do {
> > > seq = READ_ONCE(latch->seq);
> > > smp_read_barrier_depends();
> > >
> > > idx = seq & 0x01;
> > > entry = data_query(latch->data[idx], ...);
> > >
> > > smp_rmb();
> > > } while (seq != latch->seq);
> > >
>
> > Should we look at introducing yet another seq primitive?
>
> Like so?
>
> ---
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -233,6 +233,11 @@ static inline void raw_write_seqcount_en
> s->sequence++;
> }
>
> +static inline int raw_read_seqcount_latch(seqcount_t *s)
> +{
> + return lockless_dereference(s->sequence);
> +}
> +
> /**
> * raw_write_seqcount_latch - redirect readers to even/odd copy
> * @s: pointer to seqcount_t
> @@ -284,8 +289,7 @@ static inline void raw_write_seqcount_en
> * unsigned seq, idx;
> *
> * do {
> - * seq = latch->seq;
> - * smp_rmb();
> + * seq = lockless_dereference(latch->seq);
> *
> * idx = seq & 0x01;
> * entry = data_query(latch->data[idx], ...);
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -393,7 +393,7 @@ static __always_inline u64 __ktime_get_f
> u64 now;
>
> do {
> - seq = raw_read_seqcount(&tkf->seq);
> + seq = raw_read_seqcount_latch(&tkf->seq);
> tkr = tkf->base + (seq & 0x01);
> now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
> } while (read_seqcount_retry(&tkf->seq, seq));

Sounds good to me!

Thanks,

Ingo


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