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

[Petr Mladek]

On Fri 2018-11-23 11:40:48, Sergey Senozhatsky wrote:
> 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.

The best solution would be lockless console drivers. Sigh.


> 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.

Is the re-entrance safe? Some risk might be acceptable in Oops/panic
situations. It is much less acceptable for random warnings.

Best Regards,
Petr