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

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Message-ID: <20150413191750.GK23123@twins.programming.kicks-ass.net>
Date:	Mon, 13 Apr 2015 21:17:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	rusty@...tcorp.com.au, mathieu.desnoyers@...icios.com,
	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()

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);
> 

So in the actual code we use raw_read_seqcount() which includes the rmb.

This is true for the existing __ktime_get_fast_ns() as we as the new
latch_tee_find().

Should we look at introducing yet another seq primitive?

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