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Re: [PATCH v2] hw/core/qdev-clock: add a reference on aliased clocks

From: Marc-André Lureau
Subject: Re: [PATCH v2] hw/core/qdev-clock: add a reference on aliased clocks
Date: Fri, 23 Oct 2020 15:23:16 +0400


On Tue, Oct 20, 2020 at 1:11 PM Luc Michel <luc@lmichel.fr> wrote:
When aliasing a clock with the qdev_alias_clock() function, a new link
property is created on the device aliasing the clock. The link points
to the aliased clock and use the OBJ_PROP_LINK_STRONG flag. This
property is read only since it does not provide a check callback for

The object_property_add_link() documentation stats that with
OBJ_PROP_LINK_STRONG properties, the linked object reference count get
decremented when the property is deleted. But it is _not_ incremented on
creation (object_property_add_link() does not actually know the link).

This commit increments the reference count on the aliased clock to
ensure the aliased clock stays alive during the property lifetime, and
to avoid a double-free memory error when the property gets deleted.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Luc Michel <luc@lmichel.fr>

In principle, that makes sense. But I don't see any users of that API yet.

It would have been nice to have some unit tests for qdev-clock.h..

 hw/core/qdev-clock.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/hw/core/qdev-clock.c b/hw/core/qdev-clock.c
index 6a9a340d0f..eb05f2a13c 100644
--- a/hw/core/qdev-clock.c
+++ b/hw/core/qdev-clock.c
@@ -59,10 +59,18 @@ static NamedClockList *qdev_init_clocklist(DeviceState *dev, const char *name,
     } else {
         object_property_add_link(OBJECT(dev), name,
                                  (Object **) &ncl->clock,
                                  NULL, OBJ_PROP_LINK_STRONG);
+        /*
+         * Since the link property has the OBJ_PROP_LINK_STRONG flag, the clk
+         * object reference count gets decremented on property deletion.
+         * However object_property_add_link does not increment it since it
+         * doesn't know the linked object. Increment it here to ensure the
+         * aliased clock stays alive during this device life-time.
+         */
+        object_ref(OBJECT(clk));

     ncl->clock = clk;

     QLIST_INSERT_HEAD(&dev->clocks, ncl, node);

Marc-André Lureau

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