linux-kernel - Re: [PATCH] rcu: Make nocb leader kthreads process pending callbacks after spawning

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Message-ID: <20140828125841.GB4774@linux.vnet.ibm.com>
Date:	Thu, 28 Aug 2014 05:58:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Amit Shah <amit.shah@...hat.com>
Cc:	Pranith Kumar <bobby.prani@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: Make nocb leader kthreads process pending callbacks
 after spawning

On Thu, Aug 28, 2014 at 11:16:49AM +0530, Amit Shah wrote:
> On (Wed) 27 Aug 2014 [16:43:40], Pranith Kumar wrote:
> > The nocb callbacks generated before the nocb kthreads are spawned are
> > enqueued in the nocb queue for later processing. Commit fbce7497ee5af ("rcu:
> > Parallelize and economize NOCB kthread wakeups") introduced nocb leader kthreads
> > which checked the nocb_leader_wake flag to see if there were any such pending
> > callbacks. A case was reported in which newly spawned leader kthreads were not
> > processing the pending callbacks as this flag was not set, which led to a boot
> > hang.
> > 
> > The following commit ensures that the newly spawned nocb kthreads process the
> > pending callbacks by allowing the kthreads to run immediately after spawning
> > instead of waiting. This is done by inverting the logic of nocb_leader_wake
> > tests to nocb_leader_sleep which allows us to use the default initialization of
> > this flag to 0 to let the kthreads run.
> > 
> > Reported-by: Amit Shah <amit.shah@...hat.com>
> > Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> > Link: http://www.spinics.net/lists/kernel/msg1802899.html
> > ---
> >  kernel/rcu/tree.h        |  2 +-
> >  kernel/rcu/tree_plugin.h | 24 ++++++++++++------------
> >  2 files changed, 13 insertions(+), 13 deletions(-)
> 
> I'd have split this into two patches: one for the variable rename and
> one for fixing the bug.
> 
> However, the backport Paul posted does work fine for me on master, so
> you can add my
> 
> Tested-by: Amit Shah <amit.shah@...hat.com>

Thank you again, Amit, both for finding this problem and for your
testing efforts!

							Thanx, Paul

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