Messages in this thread Patch in this message | | | Date | Mon, 13 Apr 2015 21:47:25 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount_latch() |
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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));
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