linux-kernel - Re: [PATCH v2] debugobjects: Move printk out of db lock critical sections

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Date:   Mon, 17 Dec 2018 19:17:12 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Arnd Bergmann <arnd@...db.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2] debugobjects: Move printk out of db lock critical
 sections


* Waiman Long <longman@...hat.com> wrote:

> The db->lock is a raw spinlock and so the lock hold time is supposed to
> be short. This will not be the case when printk() is being involved in
> some of the critical sections.
> 
> In order to avoid the long hold time, in case some messages need to be
> printed, all the debug_object_is_on_stack() and debug_print_object()
> calls are now moved out of those critical sections in the following
> functions.
> 
>  - __debug_object_init()
>  - debug_object_activate()
>  - debug_object_deactivate()
>  - debug_object_destroy()
>  - debug_object_free()
>  - debug_object_active_state()
>  - __debug_check_no_obj_freed()
>  - check_results()
> 
> Holding the db->lock while calling printk() may lead to deadlock if
> printk() somehow requires the allocation/freeing of debug object that
> happens to be in the same hash bucket or a circular lock dependency
> warning from lockdep as reported in
> 
>   https://lkml.kernel.org/r/20181211091154.GL23332@shao2-debian

This makes me sad - whatever happened to the principle of keeping printk 
simple?

We should rename printk() to syslog() or so, and rename early_printk() to 
printk(), and be done with this.

Thanks,

	Ingo