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

[Peter Zijlstra]

On Thu, Nov 22, 2018 at 11:04:22AM +0900, Sergey Senozhatsky wrote:
> Some serial consoles call mod_timer(). So what we could have with the
> debug objects enabled was
> 
> 	mod_timer()
> 	 lock_timer_base()
> 	  debug_activate()
> 	   printk()
> 	    call_console_drivers()
> 	     foo_console()
> 	      mod_timer()
> 	       lock_timer_base()       << deadlock
> 
> That's one possible scenario. The other one can involve console's
> IRQ handler, uart port spinlock, mod_timer, debug objects, printk,
> and an eventual deadlock on the uart port spinlock. This one can
> be mitigated with printk_safe. But mod_timer() deadlock will require
> a different fix.
> 
> 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.