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

[Paul E. McKenney]

On Mon, Apr 13, 2015 at 05:08:19PM +0000, Mathieu Desnoyers wrote:
> ----- 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.

A shorthand for READ_ONCE + smp_read_barrier_depends() is the shiny
new lockless_dereference().  This helps readability, as it is often
non-trivial to work out which accesses smp_read_barrier_depends() is
helping to order.

							Thanx, Paul

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