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

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Message-ID: <20150302194613.GV15405@linux.vnet.ibm.com>
Date:	Mon, 2 Mar 2015 11:46:13 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	mingo@...nel.org, rusty@...tcorp.com.au, oleg@...hat.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 09:51:16AM +0100, Peter Zijlstra wrote:
> 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 ;-)

I do appreciate that sentiment.  ;-)

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

And, for ARM, IA64, and PowerPC, memory-barrier instructions.

							Thanx, Paul

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