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From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: paulmck@linux.vnet.ibm.com, Ed Tomlinson <edt@aei.ca>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dipankar Sarma <dipankar@in.ibm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: INFO: possible circular locking dependency detected
Date: Fri, 15 Jul 2011 12:55:57 -0400	[thread overview]
Message-ID: <1310748957.27864.62.camel@gandalf.stny.rr.com> (raw)
In-Reply-To: <1310735259.2586.330.camel@twins>

On Fri, 2011-07-15 at 15:07 +0200, Peter Zijlstra wrote:

> OK, so the latter case cannot happen (rcu_preempt_check_callbacks only
> sets NEED_QS when rcu_read_lock_nesting), we need two interrupts for
> this to happen.
> 
> rcu_read_lock()
> 
>  <IRQ>
>    |= RCU_READ_UNLOCK_NEED_QS
> 
> rcu_read_unlock()
>   __rcu_read_unlock()
>    --rcu_read_lock_nesting;
>      <IRQ>
> 	ttwu()
>           rcu_read_lock()
> 	  rcu_read_unlock()
> 	    rcu_read_unlock_special()
> 	      *BANG*
>    rcu_read_unlock_special()
> 

What about this patch? Not even compiled tested.

-- Steve

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 14dc7dd..e3545fa 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -284,18 +284,17 @@ static struct list_head *rcu_next_node_entry(struct task_struct *t,
  * notify RCU core processing or task having blocked during the RCU
  * read-side critical section.
  */
-static void rcu_read_unlock_special(struct task_struct *t)
+static int rcu_read_unlock_special(struct task_struct *t, int special)
 {
 	int empty;
 	int empty_exp;
 	unsigned long flags;
 	struct list_head *np;
 	struct rcu_node *rnp;
-	int special;
 
 	/* NMI handlers cannot block and cannot safely manipulate state. */
 	if (in_nmi())
-		return;
+		return special;
 
 	local_irq_save(flags);
 
@@ -303,7 +302,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	 * If RCU core is waiting for this CPU to exit critical section,
 	 * let it know that we have done so.
 	 */
-	special = t->rcu_read_unlock_special;
 	if (special & RCU_READ_UNLOCK_NEED_QS) {
 		rcu_preempt_qs(smp_processor_id());
 	}
@@ -311,7 +309,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	/* Hardware IRQ handlers cannot block. */
 	if (in_irq()) {
 		local_irq_restore(flags);
-		return;
+		return special;
 	}
 
 	/* Clean up if blocked during RCU read-side critical section. */
@@ -373,6 +371,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	} else {
 		local_irq_restore(flags);
 	}
+	return special;
 }
 
 /*
@@ -385,13 +384,21 @@ static void rcu_read_unlock_special(struct task_struct *t)
 void __rcu_read_unlock(void)
 {
 	struct task_struct *t = current;
+	int special;
 
+	special = ACCESS_ONCE(t->rcu_read_unlock_special);
+	/*
+	 * Clear special here to prevent interrupts from seeing it
+	 * enabled after decrementing lock_nesting and calling
+	 * rcu_read_unlock_special().
+	 */
+	t->rcu_read_unlock_special = 0;
 	barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
 	--t->rcu_read_lock_nesting;
 	barrier();  /* decrement before load of ->rcu_read_unlock_special */
-	if (t->rcu_read_lock_nesting == 0 &&
-	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
-		rcu_read_unlock_special(t);
+	if (t->rcu_read_lock_nesting == 0 && special)
+		special = rcu_read_unlock_special(t, special);
+	t->rcu_read_unlock_special = special;
 #ifdef CONFIG_PROVE_LOCKING
 	WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
 #endif /* #ifdef CONFIG_PROVE_LOCKING */



  parent reply	other threads:[~2011-07-15 16:56 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-07-14 14:49 Sergey Senozhatsky
2011-07-14 16:41 ` Peter Zijlstra
2011-07-14 16:57   ` Paul E. McKenney
2011-07-14 19:16     ` Sergey Senozhatsky
2011-07-14 19:15   ` Sergey Senozhatsky
2011-07-14 19:34     ` Paul E. McKenney
2011-07-14 19:38       ` Dave Jones
2011-07-14 20:33         ` Paul E. McKenney
2011-07-14 19:38       ` Sergey Senozhatsky
2011-07-14 16:58 ` Steven Rostedt
2011-07-14 17:02   ` Steven Rostedt
2011-07-14 17:05     ` Paul E. McKenney
2011-07-14 17:32       ` Steven Rostedt
2011-07-14 17:46         ` Steven Rostedt
2011-07-14 19:18           ` Paul E. McKenney
2011-07-14 19:41             ` Steven Rostedt
2011-07-14 20:33               ` Paul E. McKenney
2011-07-15 11:05             ` Ed Tomlinson
2011-07-15 11:29               ` Peter Zijlstra
2011-07-15 11:35                 ` Ed Tomlinson
2011-07-15 11:39                 ` Peter Zijlstra
2011-07-15 18:11                   ` Paul E. McKenney
2011-07-15 12:42                 ` Paul E. McKenney
2011-07-15 13:07                   ` Peter Zijlstra
2011-07-15 14:36                     ` Paul E. McKenney
2011-07-15 15:04                       ` Peter Zijlstra
2011-07-15 15:59                         ` Paul E. McKenney
2011-07-15 16:11                           ` Peter Zijlstra
2011-07-15 16:56                             ` Paul E. McKenney
2011-07-15 21:48                               ` Ed Tomlinson
2011-07-15 22:04                                 ` Paul E. McKenney
2011-07-16 19:42                                   ` Ed Tomlinson
2011-07-17  0:02                                     ` Paul E. McKenney
2011-07-17  1:56                                       ` Ed Tomlinson
2011-07-17 14:28                                         ` Paul E. McKenney
2011-07-18 15:15                                           ` Paul E. McKenney
2011-07-18  9:29                                     ` Peter Zijlstra
2011-07-18 15:29                                       ` Paul E. McKenney
2011-07-15 16:55                     ` Steven Rostedt [this message]
2011-07-15 17:03                       ` Paul E. McKenney
2011-07-15 17:16                         ` Steven Rostedt
2011-07-15 17:24                           ` Paul E. McKenney
2011-07-15 17:42                             ` Steven Rostedt
2011-07-15 18:33                               ` Paul E. McKenney
  -- strict thread matches above, loose matches on Subject: below --
2011-08-07 16:22 Justin P. Mattock
2011-08-11 20:57 ` Justin P. Mattock
2009-12-06 10:11 Richard Zidlicky
2009-10-10 23:09 John Kacur
2007-02-08 15:03 Pedro M. López
2006-10-16 14:05 alpha @ steudten Engineering
2006-10-16 14:32 ` Nick Piggin
2006-10-16 15:42   ` Randy Dunlap
2006-10-16 15:46     ` Nick Piggin
2006-10-19  6:02   ` Andrew Morton
2006-10-19  6:30     ` Nick Piggin

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