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

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Date:	Mon, 13 Apr 2015 11:21:46 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	George Spelvin <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 10:43 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> A shorthand for READ_ONCE + smp_read_barrier_depends() is the shiny
> new lockless_dereference()

Related side note - I think people should get used to seeing
"smp_load_acquire()". It has well-defined memory ordering properties
and should generally perform well on most architectures. It's (much)
stronger than lockless_dereference(), and together with
smp_store_release() you can make rather clear guarantees about passing
data locklessly from one CPU to another.

I'd like to see us use more of the pattern of

 - one thread does:

     .. allocate/create some data
      smp_store_release() to "expose it"

 - another thread does:

      smp_load_acquire() to read index/pointer/flag/whatever
      .. use the data any damn way you want ..

and we should probably aim to prefer that pattern over a lot of our
traditional memory barriers.

                          Linus
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