linux-kernel - Re: [PATCH tip/core/rcu 3/4] rcu: stop spurious warnings from synchronize_sched

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Message-ID: <20120221233031.GU2375@linux.vnet.ibm.com>
Date:	Tue, 21 Feb 2012 15:30:31 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH tip/core/rcu 3/4] rcu: stop spurious warnings from
 synchronize_sched_expedited

On Tue, Feb 21, 2012 at 07:57:03PM +0100, Peter Zijlstra wrote:
> On Tue, 2012-02-21 at 10:28 -0800, Paul E. McKenney wrote:
> > On Tue, Feb 21, 2012 at 07:00:17PM +0100, Peter Zijlstra wrote:
> > > On Tue, 2012-02-21 at 09:58 -0800, Paul E. McKenney wrote:
> > > > From: Hugh Dickins <hughd@...gle.com>
> > > > 
> > > > synchronize_sched_expedited() is spamming CONFIG_DEBUG_PREEMPT=y
> > > > users with an unintended warning from the cpu_is_offline() check:
> > > > use raw_smp_processor_id() instead of smp_processor_id() there.
> > > 
> > > This fails to mention why it makes sense to test a random cpu for
> > > offline-ness..
> > 
> > The check was already there, Hugh simply fixed it to use raw_.  The check
> > itself was added in c0d6d01bf (Check for illegal use of RCU from offlined
> > CPUs).  The purpose is to catch improper use of RCU from CPU_DYING
> > notifiers and on the path from the CPU_DYING notifiers to the idle loop.
> 
> Then I think this patch wants to add a comment explaining this. Because
> cpu_offline(raw_smp_processor_id()) looks really rather suspicious.

Good point, will fix!

							Thanx, Paul

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