Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic

Ville Syrjälä
2022-10-25T04:52:22Z

On Fri, Oct 21, 2022 at 08:22:41PM +0300, Ville Syrjälä wrote:
> On Mon, Aug 22, 2022 at 01:18:22PM +0200, Peter Zijlstra wrote:
> > +#ifdef CONFIG_LOCKDEP
> > + /*
> > + * It's dangerous to freeze with locks held; there be dragons there.
> > + */
> > + if (!(state & __TASK_FREEZABLE_UNSAFE))
> > + WARN_ON_ONCE(debug_locks && p->lockdep_depth);
> > +#endif
>
> We now seem to be hitting this sporadically in the intel gfx CI.
>
> I've spotted it on two machines so far:
> https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_12270/shard-tglb7/igt@gem_ctx_isolation@preservation-s3@vcs0.html
> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_109950v1/shard-snb5/igt@kms_flip@flip-vs-suspend-interruptible@a-vga1.html

Sadly no luck in reproducing this locally so far. In the meantime
I added the following patch into our topic/core-for-CI branch in
the hopes of CI stumbling on it again and dumping a bit more data:

--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -125,8 +125,16 @@ static int __set_task_frozen(struct task_struct *p, void *arg)
/*
* It's dangerous to freeze with locks held; there be dragons there.
*/
- if (!(state & __TASK_FREEZABLE_UNSAFE))
- WARN_ON_ONCE(debug_locks && p->lockdep_depth);
+ if (!(state & __TASK_FREEZABLE_UNSAFE)) {
+ static bool warned = false;
+
+ if (!warned && debug_locks && p->lockdep_depth) {
+ debug_show_held_locks(p);
+ WARN(1, "%s/%d holding locks while freezing\n",
+ p->comm, task_pid_nr(p));
+ warned = true;
+ }
+ }
#endif

WRITE_ONCE(p->__state, TASK_FROZEN);
--
2.37.4

--
Ville Syrjälä
Intel