[libvirt PATCH 2/2] util: avoid glib event loop workaround where possible

[Michal Prívozník]

On 11/25/20 7:04 PM, Daniel P. Berrangé wrote:
> I previously did a workaround for a glib event loop race
> that causes crashes:
> 
>    commit 0db4743645b7a0611a3c0687f834205c9956f7fc
>    Author: Daniel P. Berrangé <berrange at redhat.com>
>    Date:   Tue Jul 28 16:52:47 2020 +0100
> 
>      util: avoid crash due to race in glib event loop code
> 
> it turns out that the workaround has a significant performance
> penalty on I/O intensive workloads. We thus need to avoid the
> workaround if we know we have a new enough glib to avoid the
> race condition.
> 
> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
> ---
>   src/util/vireventglib.c | 29 +++++++++++++++++++++--------
>   1 file changed, 21 insertions(+), 8 deletions(-)
> 
> diff --git a/src/util/vireventglib.c b/src/util/vireventglib.c
> index 6842c6e806..8c5495bfab 100644
> --- a/src/util/vireventglib.c
> +++ b/src/util/vireventglib.c
> @@ -189,9 +189,21 @@ virEventGLibHandleFind(int watch)
>    * If the last reference to a GSource is released in a non-main
>    * thread we're exposed to a race condition that causes a
>    * crash:
> - * https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1358
> - * Thus we're using an idle func to release our ref
> + *
> + *    https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1358
> + *
> + * Thus we're using an idle func to release our ref...
> + *
> + * ...but this imposes a significant performance penalty on
> + * I/O intensive workloads which are sensitive to the iterations
> + * of the event loop, so avoid the workaround if we know we have
> + * new enough glib.
>    */
> +#if GLIB_CHECK_VERSION(2, 64, 0)
> +# define g_vir_source_unref_safe(source) g_source_unref(source)
> +#else
> +# define g_vir_source_unref_safe(source) g_idle_add(virEventGLibSourceUnrefIdle, source);

s/;//

> +

Would something like the following be totally disgusting or only a bit?

#if !GLIB_CHECK_VERSION(2, 64, 0)
# define g_source_unref(source) g_idle_add(virEventGLibSourceUnrefIdle, 
source)
#endif

- g_idle_add(...);
+ g_source_unref(...);


This way we could just drop the redefine once we upgrade min glib version.

Michal