Re: [PATCH] qom: assert integer does not overflow

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Feb 25, 2022 at 09:10:44AM -0500, Michael S. Tsirkin wrote:
> QOM reference counting is not designed with an infinite amount of
> references in mind, trying to take a reference in a loop will overflow
> the integer. We will then eventually assert when dereferencing, but the
> real problem is in object_ref so let's assert there to make such issues
> cleaner and easier to debug.

What is the actual bug / scenario that led you to hit this problem ?

I'm surprised you saw an assert in object_unref, as that would
imply you had exactly  UINT32_MAX calls to object_ref and then
one to object_unref.

> Some micro-benchmarking shows using fetch and add this is essentially
> free on x86.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  qom/object.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/qom/object.c b/qom/object.c
> index 4f0677cca9..5db3974f04 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -1167,10 +1167,14 @@ GSList *object_class_get_list_sorted(const char *implements_type,
>  Object *object_ref(void *objptr)
>  {
>      Object *obj = OBJECT(objptr);
> +    uint32_t ref;
> +
>      if (!obj) {
>          return NULL;
>      }
> -    qatomic_inc(&obj->ref);
> +    ref = qatomic_fetch_inc(&obj->ref);
> +    /* Assert waaay before the integer overflows */
> +    g_assert(ref < INT_MAX);

Not that I expect this to hit, but why choose this lower
bound instead of g_assert(ref > 0) which is the actual
failure scenario, matching the existing object_unref
assert.

Regards,
Daniel
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