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

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Message-ID: <20150302083320.GC5029@twins.programming.kicks-ass.net>
Date:	Mon, 2 Mar 2015 09:33:20 +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 Sun, Mar 01, 2015 at 02:02:23PM +0000, Mathieu Desnoyers wrote:
> > + * So during the modification, queries are first redirected to data[1]. Then we
> > + * modify data[0]. When that is complete, we redirect queries back to data[0]
> > + * and we can modify data[1].
> > + *
> > + * NOTE: The non-requirement for atomic modifications does _NOT_ include
> > + *       the publishing of new entries in the case where data is a dynamic
> > + *       data structure.
> > + *
> > + *       An iteration might start in data[0] and get suspended long enough
> > + *       to miss an entire modification sequence, once it resumes it might
> > + *       observe the new entry.
> 
> We might want to hint that in the case of dynamic data structures,
> RCU read-side C.S. and grace period should be used together with the
> latch to handle the object teardown.

Can do.

> 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 ;-)
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