linux-kernel - Re: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount

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Date:	Mon, 13 Apr 2015 17:08:19 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, rusty@...tcorp.com.au,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, rostedt@...dmis.org, tglx@...utronix.de,
	laijs@...fujitsu.com, linux@...izon.com,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>
Subject: Re: [PATCH v5 05/10] seqlock: Better document
 raw_write_seqcount_latch()

----- Original Message -----
> 
> * Peter Zijlstra <peterz@...radead.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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