Re: [PATCH] drm: fix deadlock of syncobj

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Quoting Chunming Zhou (2018-10-19 11:26:41)
> Signed-off-by: Chunming Zhou <david1.zhou@xxxxxxx>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> ---
>  drivers/gpu/drm/drm_syncobj.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 57bf6006394d..2f3c14cb5156 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -344,13 +344,16 @@ void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
>         drm_syncobj_create_signal_pt(syncobj, fence, pt_value);
>         if (fence) {
>                 struct drm_syncobj_cb *cur, *tmp;
> +               struct list_head cb_list;
> +               INIT_LIST_HEAD(&cb_list);
LIST_HEAD(cb_list); // does both in one

>                 spin_lock(&syncobj->lock);
> -               list_for_each_entry_safe(cur, tmp, &syncobj->cb_list, node) {
> +               list_splice_init(&syncobj->cb_list, &cb_list);

Steal the snapshot of the list under the lock, ok.

> +               spin_unlock(&syncobj->lock);
> +               list_for_each_entry_safe(cur, tmp, &cb_list, node) {
>                         list_del_init(&cur->node);

Races against external caller of drm_syncobj_remove_callback(). However,
it looks like that race is just fine, but we don't guard against the
struct drm_syncobj_cb itself being freed, leading to all sort of fun for
an interrupted drm_syncobj_array_wait_timeout.

That kfree seems to undermine the validity of stealing the list.
-Chris
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