linux-kernel - Re: [RFC][PATCHv3 0/6] printk: use printk_safe to handle printk() recursive calls

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Date:   Wed, 19 Oct 2016 13:33:25 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv3 0/6] printk: use printk_safe to handle printk()
 recursive calls

On (10/19/16 00:40), Sergey Senozhatsky wrote:
[..]
> Deadlock scenarios that printk_safe can handle:
> 
> a) printk recursion from logbuf_lock spin_lock section in printk()
>   printk()
>     raw_spin_lock(&logbuf_lock);
>     WARN_ON(1);
>     raw_spin_unlock(&logbuf_lock);
> 
> b) printk from sem->lock spin_lock section
>   printk()
>     console_trylock()
>       down_trylock()
>         raw_spin_lock_irqsave(&sem->lock, flags);
>         WARN_ON(1);
>         raw_spin_unlock_irqrestore(&sem->lock, flags);
> 
> c) printk from logbuf_lock spin_lock section in console_unlock()
>   printk()
>     console_unlock()
>       raw_spin_lock(&logbuf_lock);
>       WARN_ON(1);
>       raw_spin_unlock(&logbuf_lock);
> 
> d) printk from ->pi_lock from semaphore up
>   printk()
>     console_unlock()
>       up()
>         try_to_wake_up()
>           raw_spin_lock_irqsave(&p->pi_lock, flags);
>           WARN_ON(1);
>           raw_spin_unlock_irqrestore(&p->pi_lock, flags);

and

e) anything from call_console_drivers() should be fine as well.
   console_unlock() big printing loop is covered by printk_safe,
   need to protect console_cont_flush()->call_console_drivers().

   so printk "direct path" seems to be safe now:

   printk -> .. -> console_unlock() -> call_console_drivers() -> serial_console_write()

   anything that is "indirect" is not covered in this patch set.


	-ss