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

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Message-ID: <20131004111513.GA5699@redhat.com>
Date:	Fri, 4 Oct 2013 13:15:13 +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 Thu, Oct 03, 2013 at 11:10:09PM +0200, Oleg Nesterov wrote:
> > On 10/03, Oleg Nesterov wrote:
> > >
> > > So unless Peter objects I'll write the changelogs (always nontrivial task),
> > > test, and send these 2 patches + "add ops->barr() / rcu_sync_wait_for_cb"
> > > tomorrow.
> >
> > And, can't resist, probably another patch below (incomplete, needs the
> > obvious change in cpu.c and the new trivial __complete_locked() helper).
> >
> > Hmm. But when I look at wait_for_completion() I think it is buggy.

No, sorry for noise, it is fine.

> > rcusync: introduce rcu_sync_struct->exclusive mode
>
> What's exclusive to mean? One writer at a time?

Yes,

> Why don't use use the
> waitqueue in exclusive mode and use a single wake_up instead of
> wake_up_all()?

But this won't work, wait_event_exclusive(wq, CONDITION)-like code
obviously can't guarantee that only a single thread sees CONDITION,
even if "unlock" does __wake_up(nr_exclusive => 1).

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.

Oleg.

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