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

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Date:   Thu, 22 Nov 2018 11:16:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock
 critical sections

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.