linux-kernel - [Patch 2/2] cpufreq: fix a lockdep warning

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Message-Id: <20100304082721.5251.21880.sendpatchset@localhost.localdomain>
Date:	Thu, 4 Mar 2010 03:23:46 -0500
From:	Amerigo Wang <amwang@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Dave Jones <davej@...hat.com>, cpufreq@...r.kernel.org,
	Amerigo Wang <amwang@...hat.com>,
	Johannes Berg <johannes@...solutions.net>,
	akpm@...ux-foundation.org,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Subject: [Patch 2/2] cpufreq: fix a lockdep warning


There is no need to do sysfs_remove_link() or kobject_put() etc.
when policy_rwsem_write is held, move them after releasing the lock.

This fixes the lockdep warning:

halt/4071 is trying to acquire lock:
 (s_active){++++.+}, at: [<c0000000001ef868>] .sysfs_addrm_finish+0x58/0xc0

but task is already holding lock:
 (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at: [<c0000000004cd6ac>] .lock_policy_rwsem_write+0x84/0xf4

Reported-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Signed-off-by: WANG Cong <amwang@...hat.com>
Cc: Johannes Berg <johannes@...solutions.net>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Cc: Dave Jones <davej@...hat.com>

---
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 67bc2ec..e196084 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1113,6 +1113,8 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
 	unsigned int cpu = sys_dev->id;
 	unsigned long flags;
 	struct cpufreq_policy *data;
+	struct kobject *kobj;
+	struct completion *cmp;
 #ifdef CONFIG_SMP
 	struct sys_device *cpu_sys_dev;
 	unsigned int j;
@@ -1141,10 +1143,11 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
 		dprintk("removing link\n");
 		cpumask_clear_cpu(cpu, data->cpus);
 		spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
-		sysfs_remove_link(&sys_dev->kobj, "cpufreq");
+		kobj = &sys_dev->kobj;
 		cpufreq_cpu_put(data);
 		cpufreq_debug_enable_ratelimit();
 		unlock_policy_rwsem_write(cpu);
+		sysfs_remove_link(kobj, "cpufreq");
 		return 0;
 	}
 #endif
@@ -1181,7 +1184,10 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
 				data->governor->name, CPUFREQ_NAME_LEN);
 #endif
 			cpu_sys_dev = get_cpu_sysdev(j);
-			sysfs_remove_link(&cpu_sys_dev->kobj, "cpufreq");
+			kobj = &cpu_sys_dev->kobj;
+			unlock_policy_rwsem_write(cpu);
+			sysfs_remove_link(kobj, "cpufreq");
+			lock_policy_rwsem_write(cpu);
 			cpufreq_cpu_put(data);
 		}
 	}
@@ -1192,19 +1198,22 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
 	if (cpufreq_driver->target)
 		__cpufreq_governor(data, CPUFREQ_GOV_STOP);
 
-	kobject_put(&data->kobj);
+	kobj = &data->kobj;
+	cmp = &data->kobj_unregister;
+	unlock_policy_rwsem_write(cpu);
+	kobject_put(kobj);
 
 	/* we need to make sure that the underlying kobj is actually
 	 * not referenced anymore by anybody before we proceed with
 	 * unloading.
 	 */
 	dprintk("waiting for dropping of refcount\n");
-	wait_for_completion(&data->kobj_unregister);
+	wait_for_completion(cmp);
 	dprintk("wait complete\n");
 
+	lock_policy_rwsem_write(cpu);
 	if (cpufreq_driver->exit)
 		cpufreq_driver->exit(data);
-
 	unlock_policy_rwsem_write(cpu);
 
 	free_cpumask_var(data->related_cpus);
--
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