Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock critical sections

Sergey Senozhatsky
2018-11-23T02:40:55Z

On (11/22/18 11:16), Peter Zijlstra wrote:
> > So maybe we need to switch debug objects print-outs to _always_
> > printk_deferred(). Debug objects can be used in code which cannot
> > do direct printk() - timekeeping is just one example.
>
> No, printk_deferred() is a disease, it needs to be eradicated, not
> spread around.

deadlock-free printk() is deferred, but OK.

Another idea then:

---

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 70935ed91125..3928c2b2f77c 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -323,10 +323,13 @@ static void debug_print_object(struct debug_obj *obj, char *msg)
void *hint = descr->debug_hint ?
descr->debug_hint(obj->object) : NULL;
limit++;
+
+ bust_spinlocks(1);
WARN(1, KERN_ERR "ODEBUG: %s %s (active state %u) "
"object type: %s hint: %pS\n",
msg, obj_states[obj->state], obj->astate,
descr->name, hint);
+ bust_spinlocks(0);
}
debug_objects_warnings++;
}

---

This should make serial consoles re-entrant.
So printk->console_driver_write() hopefully will not deadlock.

IOW, this turns serial consoles into early_consoles, for a while ;)

-ss