linux-kernel - Re: [PATCH tip/core/rcu 41/55] rcu: Permit rt_mutex

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Date:	Mon, 19 Sep 2011 13:49:33 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	paulmck@...ux.vnet.ibm.com
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, 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 tip/core/rcu 41/55] rcu: Permit rt_mutex_unlock() with
 irqs disabled

On Mon, Sep 19, 2011 at 12:14 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Sun, Sep 18, 2011 at 12:09:23PM +0800, Yong Zhang wrote:
>> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
>> > index d3127e8..f6c63ea 100644
>> > --- a/kernel/rcutree_plugin.h
>> > +++ b/kernel/rcutree_plugin.h
>> > @@ -1149,6 +1149,8 @@ static void rcu_initiate_boost_trace(struct rcu_node *rnp)
>> >
>> >  #endif /* #else #ifdef CONFIG_RCU_TRACE */
>> >
>> > +static struct lock_class_key rcu_boost_class;
>> > +
>> >  /*
>> >   * Carry out RCU priority boosting on the task indicated by ->exp_tasks
>> >   * or ->boost_tasks, advancing the pointer to the next task in the
>> > @@ -1211,10 +1213,14 @@ static int rcu_boost(struct rcu_node *rnp)
>> >      */
>> >     t = container_of(tb, struct task_struct, rcu_node_entry);
>> >     rt_mutex_init_proxy_locked(&mtx, t);
>> > +   /* Avoid lockdep false positives.  This rt_mutex is its own thing. */
>> > +   lockdep_set_class_and_name(&mtx.wait_lock, &rcu_boost_class,
>> > +                              "rcu_boost_mutex");
>> >     t->rcu_boost_mutex = &mtx;
>>
>>       raw_spin_unlock_irqrestore(&rnp->lock, flags);  <====A
>>
>> >     rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
>> >     rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
>> > +   local_irq_restore(flags);
>>
>> Does it help here?
>> irq is enabled at A. So we still call rt_mutex_lock() with irq enabled.
>>
>> Seems should s/raw_spin_unlock_irqrestore/raw_spin_unlock ?
>
> Hmmm...  The above works at least by accident, but I am clearly not
> testing calling rt_mutex_lock(&mtx) and rt_mutex_unlock(&mtx) with
> interrupts disabled anywhere near as heavily as I thought I was.
>
> I will fix this one way or the other.

Forget to mention: if we want to suppress the lockdep warning on
overlapping usage of rcu_read_*()/local_irq_*() like below:

rcu_read_lock();
...
local_irq_disable();
...
rcu_read_unlock();
...
local_irq_enable();

'rt_mutex_unlock(rbmp);' must also be surrounded by
local_irq_irqsave()/restore().

Untested patch is attached.

Thanks,
Yong

---
From: Yong Zhang <yong.zhang0@...il.com>
Subject: [PATCH] rcu: Permit rt_mutex_unlock() with irqs disabled take#2

This make the below rcu usage really valid(AKA: lockdep
will not warn on it):

rcu_read_lock();
local_irq_disable();
rcu_read_unlock();
local_irq_enable();

Signed-off-by: Yong Zhang <yong.zhang0@...il.com>
---
 kernel/rcutree_plugin.h |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index e7eea74..d41a9b0 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -398,8 +398,11 @@ static noinline void
rcu_read_unlock_special(struct task_struct *t)

 #ifdef CONFIG_RCU_BOOST
 		/* Unboost if we were boosted. */
-		if (rbmp)
+		if (rbmp) {
+			local_irq_save(flags);
 			rt_mutex_unlock(rbmp);
+			local_irq_restore(flags);
+		}
 #endif /* #ifdef CONFIG_RCU_BOOST */

 		/*
@@ -1225,7 +1228,7 @@ static int rcu_boost(struct rcu_node *rnp)
 	lockdep_set_class_and_name(&mtx.wait_lock, &rcu_boost_class,
 				   "rcu_boost_mutex");
 	t->rcu_boost_mutex = &mtx;
-	raw_spin_unlock_irqrestore(&rnp->lock, flags);
+	raw_spin_unlock(&rnp->lock);
 	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
 	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
 	local_irq_restore(flags);
-- 
1.7.4.1

View attachment "0001-rcu-Permit-rt_mutex_unlock-with-irqs-disabled-take-2.patch" of type "text/x-patch" (1422 bytes)