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

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