The following patch fixes a deadlock experienced when devices are
being added to a bus both from a user process and eventd process.
The eventd process was hung waiting on dev->bus->subsys.rwsem which
was held by another process, which was hung since it was calling 
call_usermodehelper directly which was hung waiting for work scheduled
on the eventd workqueue to complete. The patch fixes this by delaying
the kobject_hotplug work, running it from eventd if possible. 

Backtraces of the hang:

0xc0000000017df300        1        0  0    0   D  0xc0000000017df7b0
 swapper

          SP(esp)            PC(eip)      Function(args)
0xc00000003fc9f460  0x0000000000000000  NO_SYMBOL or Userspace
0xc00000003fc9f4f0  0xc000000000058c40  .schedule +0xb4
0xc00000003fc9f5c0  0xc00000000005a464  .wait_for_completion +0x138
0xc00000003fc9f6c0  0xc00000000007c594  .call_usermodehelper +0x104
0xc00000003fc9f810  0xc00000000022d3e8  .kobject_hotplug +0x3c4
0xc00000003fc9f900  0xc00000000022d67c  .kobject_add +0x134
0xc00000003fc9f9a0  0xc00000000012b3d8  .register_disk +0x70
0xc00000003fc9fa40  0xc00000000027dfe4  .add_disk +0x60
0xc00000003fc9fad0  0xc0000000002dc7dc  .sd_probe +0x290
0xc00000003fc9fb80  0xc00000000026fbe8  .bus_match +0x94
0xc00000003fc9fc10  0xc00000000026ff70  .driver_attach +0x8c
0xc00000003fc9fca0  0xc000000000270104  .bus_add_driver +0x110
0xc00000003fc9fd50  0xc000000000270a18  .driver_register +0x38
0xc00000003fc9fdd0  0xc0000000002cd8f8  .scsi_register_driver +0x28
0xc00000003fc9fe50  0xc0000000004941d8  .init_sd +0x8c
0xc00000003fc9fee0  0xc00000000000c720  .init +0x25c
0xc00000003fc9ff90  0xc0000000000183ec  .kernel_thread +0x4c


0xc00000003fab3380        4        1  0    0   D  0xc00000003fab3830 
 events/0

          SP(esp)            PC(eip)      Function(args)
0xc00000003faaf6e0  0x0000000000000000  NO_SYMBOL or Userspace
0xc00000003faaf770  0xc000000000058c40  .schedule +0xb4
0xc00000003faaf840  0xc00000000022fa20  .rwsem_down_write_failed +0x14c
0xc00000003faaf910  0xc00000000026fed0  .bus_add_device +0x11c
0xc00000003faaf9b0  0xc00000000026e288  .device_add +0xd0
0xc00000003faafa50  0xc0000000002cdb00  .scsi_sysfs_add_sdev +0x8c
0xc00000003faafb00  0xc0000000002cbff8  .scsi_probe_and_add_lun +0xb04
0xc00000003faafc00  0xc0000000002ccca0  .scsi_add_device +0x90
0xc00000003faafcb0  0xc0000000002d9458  .ipr_worker_thread +0xc60
0xc00000003faafdc0  0xc00000000007cd9c  .worker_thread +0x268
0xc00000003faafee0  0xc0000000000839cc  .kthread +0x160
0xc00000003faaff90  0xc0000000000183ec  .kernel_thread +0x4c


---


diff -puN lib/kobject.c~kobject_hotplug_hang lib/kobject.c
--- linux-2.6.5/lib/kobject.c~kobject_hotplug_hang	Wed Apr  7 15:48:14 2004
+++ linux-2.6.5-bjking1/lib/kobject.c	Wed Apr  7 16:48:56 2004
@@ -103,8 +103,14 @@ static void fill_kobj_path(struct kset *
 static unsigned long sequence_num;
 static spinlock_t sequence_lock = SPIN_LOCK_UNLOCKED;
 
-static void kset_hotplug(const char *action, struct kset *kset,
-			 struct kobject *kobj)
+struct hotplug_work {
+	struct work_struct work;
+	const char *action;
+	struct kset *kset;
+	struct kobject *kobj;
+};
+
+static void kset_hotplug_work(void *data)
 {
 	char *argv [3];
 	char **envp = NULL;
@@ -116,22 +122,26 @@ static void kset_hotplug(const char *act
 	char *kobj_path = NULL;
 	char *name = NULL;
 	unsigned long seq;
+	struct hotplug_work *work = (struct hotplug_work *)data;
+	const char *action = work->action;
+	struct kset *kset = work->kset;
+	struct kobject *kobj = work->kobj;
 
 	/* If the kset has a filter operation, call it. If it returns
-	   failure, no hotplug event is required. */
+	 failure, no hotplug event is required. */
 	if (kset->hotplug_ops->filter) {
 		if (!kset->hotplug_ops->filter(kset, kobj))
-			return;
+			goto exit;
 	}
 
 	pr_debug ("%s\n", __FUNCTION__);
 
 	if (!hotplug_path[0])
-		return;
+		goto exit;
 
 	envp = kmalloc(NUM_ENVP * sizeof (char *), GFP_KERNEL);
 	if (!envp)
-		return;
+		goto exit;
 	memset (envp, 0x00, NUM_ENVP * sizeof (char *));
 
 	buffer = kmalloc(BUFFER_SIZE, GFP_KERNEL);
@@ -176,8 +186,8 @@ static void kset_hotplug(const char *act
 	if (kset->hotplug_ops->hotplug) {
 		/* have the kset specific function add its stuff */
 		retval = kset->hotplug_ops->hotplug (kset, kobj,
-				  &envp[i], NUM_ENVP - i, scratch,
-				  BUFFER_SIZE - (scratch - buffer));
+						     &envp[i], NUM_ENVP - i, scratch,
+						     BUFFER_SIZE - (scratch - buffer));
 		if (retval) {
 			pr_debug ("%s - hotplug() returned %d\n",
 				  __FUNCTION__, retval);
@@ -193,10 +203,40 @@ static void kset_hotplug(const char *act
 			  __FUNCTION__, retval);
 
 exit:
+	kset_put(kset);
+	kobject_put(kobj);
 	kfree(kobj_path);
+	kfree(work);
 	kfree(buffer);
 	kfree(envp);
-	return;
+}
+
+static void kset_hotplug(const char *action, struct kset *kset,
+			 struct kobject *kobj)
+{
+	struct hotplug_work *work;
+
+	if (!(work = kmalloc(sizeof(*work), GFP_KERNEL)))
+		return;
+
+	work->action = action;
+	if (!(work->kset = kset_get(kset))) {
+		kfree(work);
+		return;
+	}
+
+	if (!(work->kobj = kobject_get(kobj))) {
+		kset_put(kset);
+		kfree(work);
+		return;
+	}
+
+	INIT_WORK(&work->work, kset_hotplug_work, work);
+
+	if (keventd_up())
+		schedule_work(&work->work);
+	else
+		kset_hotplug_work(work);
 }
 
 void kobject_hotplug(const char *action, struct kobject *kobj)

_