linux-kernel - Re: [PATCH RFC tip/core/rcu 28/28] rcu: Fix idle-task checks

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Date:	Thu, 3 Nov 2011 14:00:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.org, "Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 28/28] rcu: Fix idle-task checks

On Wed, Nov 02, 2011 at 09:55:09PM -0700, Josh Triplett wrote:
> On Wed, Nov 02, 2011 at 01:30:49PM -0700, Paul E. McKenney wrote:
> > From: Paul E. McKenney <paul.mckenney@...aro.org>
> > 
> > RCU has traditionally relied on idle_cpu() to determine whether a given
> > CPU is running in the context of an idle task, but recent changes have
> > invalidated this approach.  This commit therefore switches from idle_cpu
> > to "current->pid != 0".
> 
> Could you elaborate a bit on "recent changes"?  It looks like you mean
> commit 908a3283728d92df36e0c7cd63304fd35e93a8a9; if so, could you add
> that reference to the commit message?

Will do!

> Also, the hard-coded use of "current->pid != 0" concerns me.  Could this
> use some existing function?  Does idle_task() help?  If no appropriate
> predicate exists, perhaps it should.  is_idle_task(current)?

I could use idle_task(), but that does quite a bit more work.
The hard-coded "current->pid != 0" is used in a number of other places
in the kernel, so there is precedent.  Might be worth fixing globally
as a separate fix, though.

							Thanx, Paul

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