linux-kernel - Re: [PATCH tip v4 4/5] rcu: Do not call rcu_nocb_gp

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Date:	Wed, 25 Nov 2015 11:29:05 +0100
From:	Daniel Wagner <daniel.wagner@...-carit.de>
To:	Boqun Feng <boqun.feng@...il.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-rt-users@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH tip v4 4/5] rcu: Do not call rcu_nocb_gp_cleanup() while
 holding rnp->lock

Hi Boqun,

On 11/25/2015 02:01 AM, Boqun Feng wrote:
> On Tue, Nov 24, 2015 at 11:52:12PM +0800, Boqun Feng wrote:
>> Hi Daniel,
>>
>> On Tue, Nov 24, 2015 at 02:03:06PM +0100, Daniel Wagner wrote:
>>> rcu_nocb_gp_cleanup() is called while holding rnp->lock. Currently,
>>> this is okay because the wake_up_all() in rcu_nocb_gp_cleanup() will
>>> not enable the IRQs. lockdep is happy.
>>>
>>> By switching over using swait this is not true anymore. swake_up_all()
>>> enables the IRQs while processing the waiters. __do_softirq() can now
>>> run and will eventually call rcu_process_callbacks() which wants to
>>> grap nrp->lock.
>>>
>>> Let's move the rcu_nocb_gp_cleanup() call outside the lock before we
>>> switch over to swait.
>>>
>>
>> But you did introduce swait in this patch ;-)

Argh, that is a fail. I did build all patches individual but seems like
I haven't got the right configuration.

>> [snip]
>>
>>>
>>> Signed-off-by: Daniel Wagner <daniel.wagner@...-carit.de>
>>> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Cc: linux-kernel@...r.kernel.org
>>> ---
>>>  kernel/rcu/tree.c        |  4 +++-
>>>  kernel/rcu/tree.h        |  3 ++-
>>>  kernel/rcu/tree_plugin.h | 16 +++++++++++++---
>>>  3 files changed, 18 insertions(+), 5 deletions(-)
>>>
>>
>> So I tried to build this patch with a config having RCU_EXPERT=y and
>> RCU_NOCB_CPU=y, but I got:

Will update my config accordingly.

Thanks,
Daniel
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