On Mon, 26 Aug 2024 at 16:22, Peter Xu <pet...@redhat.com> wrote: > > On Fri, Aug 23, 2024 at 03:13:11PM +0900, Akihiko Odaki wrote: > > memory_region_update_container_subregions() used to call > > memory_region_ref(), which creates a reference to the owner of the > > subregion, on behalf of the owner of the container. This results in a > > circular reference if the subregion and container have the same owner. > > > > memory_region_ref() creates a reference to the owner instead of the > > memory region to match the lifetime of the owner and memory region. We > > do not need such a hack if the subregion and container have the same > > owner because the owner will be alive as long as the container is. > > Therefore, create a reference to the subregion itself instead ot its > > owner in such a case; the reference to the subregion is still necessary > > to ensure that the subregion gets finalized after the container. > > > > Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com> > > --- > > system/memory.c | 8 ++++++-- > > 1 file changed, 6 insertions(+), 2 deletions(-) > > > > diff --git a/system/memory.c b/system/memory.c > > index 5e6eb459d5de..e4d3e9d1f427 100644 > > --- a/system/memory.c > > +++ b/system/memory.c > > @@ -2612,7 +2612,9 @@ static void > > memory_region_update_container_subregions(MemoryRegion *subregion) > > > > memory_region_transaction_begin(); > > > > - memory_region_ref(subregion); > > + object_ref(mr->owner == subregion->owner ? > > + OBJECT(subregion) : subregion->owner); > > The only place that mr->refcount is used so far is the owner with the > object property attached to the mr, am I right (ignoring name-less MRs)? > > I worry this will further complicate refcounting, now we're actively using > two refcounts for MRs.. > > Continue discussion there: > > https://lore.kernel.org/r/067b17a4-cdfc-4f7e-b7e4-28c38e1c1...@daynix.com > > What I don't see is how mr->subregions differs from mr->container, so we > allow subregions to be attached but not the container when finalize() > (which is, afaict, the other way round). > > It seems easier to me that we allow both container and subregions to exist > as long as within the owner itself, rather than start heavier use of > mr->refcount.
I don't think just "same owner" necessarily will be workable -- you can have a setup like: * device A has a container C_A * device A has a child-device B * device B has a memory region R_B * device A's realize method puts R_B into C_A R_B's owner is B, and the container's owner is A, but we still want to be able to get rid of A (in the process getting rid of B because it gets unparented and unreffed, and R_B and C_A also). -- PMM
