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Date: Fri, 4 Oct 2013 13:50:21 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] rcu: Create rcu_sync infrastructure
Message-ID: <20131004115021.GA7207@redhat.com>
References: <20131003184719.GA11996@redhat.com> <20131003192135.GR5790@linux.vnet.ibm.com> <20131003193206.GA17796@redhat.com> <20131003193319.GB17796@redhat.com> <20131003195026.GT5790@linux.vnet.ibm.com> <20131003200002.GA23768@redhat.com> <20131003211009.GA4127@redhat.com> <20131004071814.GS28601@twins.programming.kicks-ass.net> <20131004111513.GA5699@redhat.com> <20131004113607.GI3081@twins.programming.kicks-ass.net>
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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.