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

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Date:	Mon, 13 Apr 2015 18:32:02 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.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()


* Peter Zijlstra <peterz@...radead.org> wrote:

> +/**
>   * raw_write_seqcount_latch - redirect readers to even/odd copy
>   * @s: pointer to seqcount_t
> + *
> + * The latch technique is a multiversion concurrency control method that allows
> + * queries during non atomic modifications. If you can guarantee queries never
> + * interrupt the modification -- e.g. the concurrency is strictly between CPUs
> + * -- you most likely do not need this.

Speling nit:

 triton:~/tip> git grep -i 'non-atomic' | wc -l
 160
 triton:~/tip> git grep -i 'non atomic' | wc -l
 21

so I guess 'non-atomic' wins?

> + *
> + * Where the traditional RCU/lockless data structures rely on atomic
> + * modifications to ensure queries observe either the old or the new state the
> + * latch allows the same for non atomic updates. The trade-off is doubling the
> + * cost of storage; we have to maintain two copies of the entire data
> + * structure.

s/non atomic/non-atomic

> + * 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().)

Thanks,

	Ingo
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