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

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Date:	Fri, 4 Oct 2013 21:06:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 10/04, Peter Zijlstra wrote:
>
> On Fri, Oct 04, 2013 at 05:13:23PM +0200, Oleg Nesterov wrote:
>
> > 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.

OK, I see your point.

But rcu_sync_struct has to serialize the writers anyway. The only
question is how many other writers the thread doing ->sync() should
wakeup and when.

And otoh. Currently nobody needs the non-exclusive mode (cpu-hotplug
doesn't care because it is always exclusive itself). And in fact you
initially argued with wake_up_all ;) "exclusive" is more natural, it
is like rw_semaphore.

However, yes-yes-yes, I do think that we need the non-exclusive mode
too, at least for percpu_down_write_nonexclusive() which I think we
need as well.

Oleg.

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