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

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH] hw/core/qdev-clock: add a reference on aliased clocks
Date: Tue, 20 Oct 2020 10:32:26 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1

On 10/20/20 9:58 AM, Philippe Mathieu-Daudé wrote:
Cc'ing Markus/Marc-André too.

On 10/20/20 9:44 AM, Luc Michel 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 get deleted.

Signed-off-by: Luc Michel <luc@lmichel.fr>
  hw/core/qdev-clock.c | 1 +
  1 file changed, 1 insertion(+)

diff --git a/hw/core/qdev-clock.c b/hw/core/qdev-clock.c
index 6a9a340d0f..5f5e143702 100644
--- a/hw/core/qdev-clock.c
+++ b/hw/core/qdev-clock.c
@@ -59,10 +59,11 @@ 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);
+        object_ref(OBJECT(clk));

OK, this is particular to this model because device_finalize()
garbage-collector and calls qdev_finalize_clocklist().

With a comment explaining why we need this call:
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

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

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