Re: [Qemu-devel] [PATCH v4 6/7] qom: replace object property list with GHashTable

["Andreas Färber" <afaerber@suse.de>]

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Am 16.11.2015 um 10:38 schrieb Andreas Färber:
> Am 16.11.2015 um 09:16 schrieb Christian Borntraeger:
>> On 11/16/2015 08:13 AM, Pavel Fedin wrote:
>>>>>> (process:4102): GLib-CRITICAL **: g_hash_table_iter_next: assertion
>>>>>> 'ri->version == ri->hash_table->version' failed
>>>>>>
>>>>>> (process:4102): GLib-CRITICAL **: g_hash_table_iter_next: assertion
>>>>>> 'ri->version == ri->hash_table->version' failed
>>>>>>
>>>>>> (process:4102): GLib-CRITICAL **: iter_remove_or_steal: assertion
>>>>>> 'ri->version == ri->hash_table->version' failed
>>>
>>>  Wow... Actually this may come from attempts to modify the tree inside iteration.
>>>
>>>> Thanks! sclp_init() seems to violate several QOM design principles in
>>>> that it uses object_new() during TypeInfo::instance_init() and uses a
>>>> TYPE_... constant as property name. But nothing else stands out immediately.
>>>
>>>  I think we should refactor this and retry. If not all problems go away, then we are indeed modifying the tree during iteration, and
>>> we have to find some solution.
>>
>> David, Conny,
>>
>> the current tree of afaerber
>>
>> https://github.com/afaerber/qemu-cpu/commits/qom-next
>>
>> has this patch:
>>
>>> From: Pavel Fedin <p.fedin@samsung.com>
>>>
>>> ARM GICv3 systems with large number of CPUs create lots of IRQ pins. Since
>>> every pin is represented as a property, number of these properties becomes
>>> very large. Every property add first makes sure there's no duplicates.
>>> Traversing the list becomes very slow, therefore qemu initialization takes
>>> significant time (several seconds for e. g. 16 CPUs).
>>>
>>> This patch replaces list with GHashTable, making lookup very fast. The only
>>> drawback is that object_child_foreach() and object_child_foreach_recursive()
>>> cannot modify their objects during traversal, since GHashTableIter does not
>>> have modify-safe version. However, the code seems not to modify objects via
>>> these functions.
>>>
>>> Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
>>> Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
>>
>> which causes failures in make check. A simple reproducer is
>>
>> qemu-system-s390x -device sclp,help
>>
>>
>> any idea what would be the most simple fix?
>> Can we refactor this to create the event facility and the bus in the
>> machine or whatever?
> 
> I believe it is rather a very general problem with the new
> object_property_del_all() implementation. It iterates through
> properties, releasing child<> and link<> properties, which results in an
> unref, which at some point unparents that device, removing it in the
> parent's properties hashtable while the parent is iterating through it.
> 
> In this case it seems to be about the bus child<> on the event facility.
> 
>>>  I wonder... Could we have both list and hashtable? hashtable for searching by name and list for iteration. In this case we would
>>> not have to use glib's iterators, and would be free of problems with them. Just keep the list and hashtable in sync.
>>>  Or, is there any hashtable implementation out there which would keep iterators valid during modification?
>>>  OTOH, glib has a function "remove the element at iterator's position", and we could postpone addition. So, perhaps, using both
>>> containers would be an overkill, just refactor the code to adapt to the new behavior.
> 
> My idea, which I wanted to investigate after the weekend, is iterating
> through the hashtable to create a list of prop->release functions and
> call them only after finishing the iteration. That might not work
> either, so we may need to loop over the releasing to allow for released
> properties to disappear after prop->release().

I went with the latter and squashed the attached fixup (without last two
hunks, preparing a separate patch for that), interrupting each iteration
after prop->release() to be safe. That seems to fix it.

Will prepend and test Dan's unit test next.

Thanks,
Andreas

-- 
SUSE Linux GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Felix Imendörffer, Jane Smithard, Graham Norton; HRB 21284 (AG Nürnberg)

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diff --git a/qom/object.c b/qom/object.c
index 0ac3bc1..284fa38 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -377,14 +377,22 @@ static void object_property_del_all(Object *obj)
     ObjectProperty *prop;
     GHashTableIter iter;
     gpointer key, value;
+    bool released;
 
-    g_hash_table_iter_init(&iter, obj->properties);
-    while (g_hash_table_iter_next(&iter, &key, &value)) {
-        prop = value;
-        if (prop->release) {
-            prop->release(obj, prop->name, prop->opaque);
+    do {
+        released = false;
+        g_hash_table_iter_init(&iter, obj->properties);
+        while (g_hash_table_iter_next(&iter, &key, &value)) {
+            prop = value;
+            if (prop->release) {
+                prop->release(obj, prop->name, prop->opaque);
+                prop->release = NULL;
+                released = true;
+                break;
+            }
+            g_hash_table_iter_remove(&iter);
         }
-    }
+    } while (released);
 
     g_hash_table_unref(obj->properties);
 }
@@ -401,7 +409,15 @@ static void object_property_del_child(Object *obj, Object *child, Error **errp)
         if (object_property_is_child(prop) && prop->opaque == child) {
             if (prop->release) {
                 prop->release(obj, prop->name, prop->opaque);
+                prop->release = NULL;
             }
+            break;
+        }
+    }
+    g_hash_table_iter_init(&iter, obj->properties);
+    while (g_hash_table_iter_next(&iter, &key, &value)) {
+        prop = value;
+        if (object_property_is_child(prop) && prop->opaque == child) {
             g_hash_table_iter_remove(&iter);
             break;
         }
@@ -856,7 +872,7 @@ void object_ref(Object *obj)
     if (!obj) {
         return;
     }
-     atomic_inc(&obj->ref);
+    atomic_inc(&obj->ref);
 }
 
 void object_unref(Object *obj)
@@ -864,7 +880,7 @@ void object_unref(Object *obj)
     if (!obj) {
         return;
     }
-    g_assert(obj->ref > 0);
+    g_assert_cmpint(obj->ref, >, 0);
 
     /* parent always holds a reference to its children */
     if (atomic_fetch_dec(&obj->ref) == 1) {