linux-kernel - Re: [PATCH 2/3] rcu: Create rcu

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Date:	Fri, 4 Oct 2013 18:25:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] rcu: Create rcu_sync infrastructure

On Fri, Oct 04, 2013 at 05:13:23PM +0200, Oleg Nesterov wrote:
> Not sure I understand...
> 
> percpu-rwsem will use exclusive mode, with or without the possible
> improvements we can copy from cpuhp. sb_writers can probably use it
> too (along with other helpers from percpu-rwsem we do not have yet).

Oh; I thought to have understood it would go away when you switched to
the reader scheme from the hotplug bits.

But thinking a little more; yes you'd still need something to serialize
writers.

> If you mean that the "raw" rcu_sync infrastructure will never have
> more users, will I am not sure, it looks "natural". But I can not
> know for sure.

Right, so this would be something I'd forgo if there were no immediate
users. We could always reconsider if there was one; but apparently
percpu-rwsem is one..

> I am not trying saying this feature is "must have", of course it
> is not. The only problem, I am a bit puzzled why you dislike it
> that much.

The reason I dislike it is because I feel we're now mixing two objects
into one; one object doing mutual exclusion and one object being
terribly smart with sync_rcu.

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