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Date: Wed, 19 Oct 2016 13:33:25 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>, Calvin Owens <calvinowens@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [RFC][PATCHv3 0/6] printk: use printk_safe to handle printk()
 recursive calls
Message-ID: <20161019043325.GA444@swordfish>
References: <20161018154045.7364-1-sergey.senozhatsky@gmail.com>
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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