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

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Message-ID: <20131004112945.GB5699@redhat.com>
Date:	Fri, 4 Oct 2013 13:29:45 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 10/03, Paul E. McKenney wrote:
>
> On Thu, Oct 03, 2013 at 11:10:09PM +0200, Oleg Nesterov wrote:
> >
> > rcusync: introduce rcu_sync_struct->exclusive mode
> >
> > CHANGELOG.
>
> Should the changelog really be in all caps?  (Sorry, couldn't resist...)

Apparently you do not realize it is going to be an EXCELLENT changelog!

> > @@ -53,9 +55,13 @@ void rcu_sync_enter(struct rcu_sync_struct *rss)
> >  	if (need_sync) {
> >  		rss->ops->sync();
> >  		rss->gp_state = GP_PASSED;
> > -		wake_up_all(&rss->gp_wait);
> > +		if (!rss->exclusive)
> > +			wake_up_all(&rss->gp_comp.wait);
>
> Not sure about the wake_up_all() on a completion,

Yes, we reuse completion->wait in the !exclusive case. Like we reuse
its spinlock as rss_lock.

We can add a completion/complete union, but this will complicate the
code a bit and imo doesn't make sense.

> but if we are exclusive,
> don't we need to complete() the completion here?

No, if we are exclusive we should delay the "wake up the next writer"
till rcu_sync_exit().

> Oh, I guess gp_comp.wait is exactly a wait_queue_head_t, so I guess
> you can do wake_up_all() on it...

Yes, and we never "mix" completion/wait_queue_head_t operations/members.
IOW, we always use ->gp_comp if "exclusive", and only ->gp_comp.wait is
used otherwise.

> Never mind!!!

Agreed ;)

Oleg.

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