linux-kernel - Re: [RFC][PATCH 6/9] seqlock: Better document raw_write_seqcount

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Message-ID: <20150302085116.GD24151@twins.programming.kicks-ass.net>
Date:	Mon, 2 Mar 2015 09:51:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	mingo@...nel.org, rusty@...tcorp.com.au, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, rostedt@...dmis.org, tglx@...utronix.de,
	Michel Lespinasse <walken@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH 6/9] seqlock: Better document
 raw_write_seqcount_latch()

On Mon, Mar 02, 2015 at 09:33:20AM +0100, Peter Zijlstra wrote:
> On Sun, Mar 01, 2015 at 02:02:23PM +0000, Mathieu Desnoyers wrote:

> > The latch, AFAIU, takes care of making sure the new objects are
> > initialized before being published into the data structure, so there
> > would be no need to use RCU assign pointer. However, we really need
> > RCU around reads, along with a grace period between removal of an object
> > and its teardown.
> 
> So I do need the rcu_assign_pointer for the RB link because that also
> initializes the rb_node itself. Or put differently, be _very_ _VERY_
> sure your entire object is initialized before the latch.
> 
> Secondly, note that the latch does a WMB and rcu_assign_pointer does a
> RELEASE, these are not equivalent.
> 
> So I don't think I will highlight this particular point. If you're sure
> enough to know the difference you can get away with it, sure. But in
> general I think people should still use rcu_assign_pointer; if only to
> make Paul sleep better at night ;-)

Also note that if you do not use rcu_assign_pointer() one will at the
very least require WRITE_ONCE().
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