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

[Mathieu Desnoyers]

----- Original Message -----
> 
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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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