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

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Date:	Fri, 4 Oct 2013 13:50:21 +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 01:15:13PM +0200, Oleg Nesterov wrote:
> > Of course we can fix this, but wait_for_completion/complete already
> > does the necessary work: x->done acts as a resource counter which is
> > always checked/incremented/decremented under the same lock.
>
> Urgh, so now you're not using a semaphore as completion but using a
> completion as semaphore.

Yes. And we can use it as semaphore only because we already have gp_count.

Of course, we could add ->gp_mutex and change enter/exit to lock/unlock
it if exclusive. But I do not really like it even if this can makes the
code a bit (only a little bit) simpler. This will increase sizeof() and
for no reason, I think, because to some degree this will also duplicate
the synchronization logic. And while this doesn't really matter, this
will penalize the contending writers a bit: it is not easy (but of
course possible) to take this mutex after rss->gp_count++, and we do
want to increment this counter "asap".

Oleg.

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