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SubjectRe: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount_latch()
----- Original Message -----
>
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
[...]
> > + * The query will have a form like:
> > + *
> > + * struct entry *latch_query(struct latch_struct *latch, ...)
> > + * {
> > + * struct entry *entry;
> > + * unsigned seq, idx;
> > + *
> > + * do {
> > + * seq = latch->seq;
> > + * smp_rmb();
> > + *
> > + * idx = seq & 0x01;
> > + * entry = data_query(latch->data[idx], ...);
> > + *
> > + * smp_rmb();
> > + * } while (seq != latch->seq);
>
> 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);
>
> Note that there's just a single smp_rmb() barrier: the READ_ONCE() is
> there to make sure GCC doesn't calculate 'idx' from two separate
> reads, but otherwise there's a direct data dependency on latch->seq so
> no smp_rmb() is needed, only a data dependency barrier when doing the
> first lookup AFAICS?
>
> (This doesn't matter on x86 where smp_rmb() is barrier().)

The latch evolved from seqlock.h, where there was no
data dependency between the sequence counter and the
data read, hence the smp_rmb(). Indeed, there is a
data dependency in the case of the latch, so I think
your approach of READ_ONCE + smp_read_barrier_depends()
is appropriate.

Thanks!

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com


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