From: Venkatesh Srinivas <me@endeavour.zapto.org>
Date: Sun, 27 Mar 2011 14:20:20 +0000 (-0700)
Subject: kernel -- vm_object locking: DEBUG_LOCKS check for hold_wait vs hold deadlock
X-Git-Tag: v2.10.0~137
X-Git-Url: http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff_plain/cb443cbb47dab690574674c82ee9407c28deb544

kernel -- vm_object locking: DEBUG_LOCKS check for hold_wait vs hold deadlock

If a thread has a hold on a vm_object and enters hold_wait (via either
vm_object_terminate or vm_object_collapse), it will wait forever for the hold
count to hit 0. Record the threads holding an object in a per-object array.
---

diff --git a/sys/vm/vm_object.c b/sys/vm/vm_object.c
index 499a0a4..e227ccd 100644
--- a/sys/vm/vm_object.c
+++ b/sys/vm/vm_object.c
@@ -1819,6 +1819,14 @@ vm_object_set_writeable_dirty(vm_object_t object)
 static void
 vm_object_lock_init(vm_object_t obj)
 {
+#if defined(DEBUG_LOCKS)
+	int i;
+
+	obj->debug_hold_bitmap = 0;
+	for (i = 0; i < VMOBJ_DEBUG_ARRAY_SIZE; i++) {
+		obj->debug_hold_thrs[i] = NULL;
+	}
+#endif
 }
 
 void
@@ -1839,6 +1847,18 @@ vm_object_hold(vm_object_t obj)
 	vm_object_lock(obj);
 
 	refcount_acquire(&obj->hold_count);
+
+#if defined(DEBUG_LOCKS)
+	int i;
+
+	i = ffs(~obj->debug_hold_bitmap) - 1;
+	if (i == -1) {
+		panic("vm_object hold count > VMOBJ_DEBUG_ARRAY_SIZE");
+	}
+
+	obj->debug_hold_bitmap |= (1 << i);
+	obj->debug_hold_thrs[i] = curthread;
+#endif
 }
 
 void
@@ -1846,6 +1866,24 @@ vm_object_drop(vm_object_t obj)
 {
 	int rc;
 
+#if defined(DEBUG_LOCKS)
+	int found = 0;
+	int i;
+
+	for (i = 0; i < VMOBJ_DEBUG_ARRAY_SIZE; i++) {
+		if ((obj->debug_hold_bitmap & (1 << i)) &&
+		    (obj->debug_hold_thrs[i] == curthread)) {
+			obj->debug_hold_bitmap &= ~(1 << i);
+			obj->debug_hold_thrs[i] = NULL;
+			found = 1;
+			break;
+		}
+	}
+
+	if (found == 0)
+		panic("vm_object: attempt to drop hold on non-self-held obj");
+#endif
+
 	rc = refcount_release(&obj->hold_count);
 	vm_object_unlock(obj);
 
@@ -1864,6 +1902,16 @@ vm_object_hold_wait(vm_object_t obj)
 {
 	vm_object_lock(obj);
 
+#if defined(DEBUG_LOCKS)
+	int i;
+
+	for (i = 0; i < VMOBJ_DEBUG_ARRAY_SIZE; i++) {
+		if ((obj->debug_hold_bitmap & (1 << i)) &&
+		    (obj->debug_hold_thrs[i] == curthread))
+			panic("vm_object: self-hold in terminate or collapse");
+	}
+#endif
+
 	while (obj->hold_count)
 		tsleep(obj, 0, "vmobjhld", 0);
 
diff --git a/sys/vm/vm_object.h b/sys/vm/vm_object.h
index cb29529..68769f2 100644
--- a/sys/vm/vm_object.h
+++ b/sys/vm/vm_object.h
@@ -147,6 +147,17 @@ struct vm_object {
 	void *handle;
 	int hold_count;			/* refcount for object liveness */
 	
+#if defined(DEBUG_LOCKS)
+	/* 
+	 * Record threads holding a vm_object
+	 */
+
+#define VMOBJ_DEBUG_ARRAY_SIZE		(32)
+	u_int debug_hold_bitmap;
+	thread_t debug_hold_thrs[VMOBJ_DEBUG_ARRAY_SIZE];
+
+#endif
+
 	union {
 		/*
 		 * Device pager