Re: rcu, sched: WARNING: CPU: 30 PID: 23771 at kernel/rcu/tree_plugin.h:337 rcu_read_unlock_special+0x369/0x550()

From: Sasha Levin <sasha.levin@oracle.com>
To: paulmck@linux.vnet.ibm.com, Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"davej@codemonkey.org.uk >> Dave Jones" <davej@codemonkey.org.uk>
Subject: Re: rcu, sched: WARNING: CPU: 30 PID: 23771 at kernel/rcu/tree_plugin.h:337 rcu_read_unlock_special+0x369/0x550()
Date: Tue, 27 Jan 2015 17:08:21 -0500	[thread overview]
Message-ID: <54C80C55.2070908@oracle.com> (raw)
In-Reply-To: <20150127220329.GF19109@linux.vnet.ibm.com>

On 01/27/2015 05:03 PM, Paul E. McKenney wrote:
> On Mon, Jan 26, 2015 at 10:08:04AM +0800, Lai Jiangshan wrote:
>> > On 01/25/2015 05:18 AM, Paul E. McKenney wrote:
>> > 
>>> > > 
>>> > > Good point!  In my scenario, CPU 0 would not yet have switched away from
>>> > > Task A.  Hmmm...  Yet Sasha really does see this failure.  Will give it
>>> > > some more thought.
>>> > > 
>>> > > Any ideas?
>> > 
>> > I don't known which commit was merged from the rcu-git-tree in Sasha's test
>> > I try to review it.
> If I had to guess, it would be 1d082fd06188 (Remove local_irq_disable()
> in rcu_preempt_note_context_switch()), though his finding this might be
> more directly related to increases in trinity's levels of stress.

Quick update from my end: I've stopped seeing this warning, but I've also stopped
seeing warnings for the other RCU issue I've reported (https://lkml.org/lkml/2015/1/22/676)
so I'm slightly unhappy about that.

>> > We can fallback to git-bitsect if the reviews fails.
> One (very unlikely) possibility is that Sasha's compiler is ignoring the
> barrier() in rcu_preempt_qs().

I'm actually running the latest gcc (trunk) as well, so it's very possible that it was
doing something stupid.

Thanks,
Sasha