linux-kernel - Re: [PATCH 2/2 v3] lib: debugobjects: touch watchdog to avoid softlockup when !CONFIG

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Date:   Wed, 17 Jan 2018 13:21:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Yang Shi <yang.s@...baba-inc.com>
cc:     longman@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2 v3] lib: debugobjects: touch watchdog to avoid
 softlockup when !CONFIG_PREEMPT

On Thu, 4 Jan 2018, Yang Shi wrote:

> There are nested loops on debug objects free path, sometimes it may take
> over hundred thousands of loops, then cause soft lockup with !CONFIG_PREEMPT
> occasionally, like below:

Please trim back traces. The whole module info and whatever is completely
irrelevant.

> @@ -768,6 +771,10 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size)
>  			debug_objects_maxchain = cnt;
>  
>  		max_loops += cnt;
> +
> +		if (max_loops > 10000 && ((max_loops % 10000) == 0)
> +		    && suppress_lockup != 0)
> +			touch_softlockup_watchdog();

This is voodoo programming.

There are two things which can be done here:

 1) The collected objects can be put on a global free list and work
    scheduled to free them piecewise.

 2) We can do a cond_resched() if not in atomic context and interrupts are
    enabled.

I rather prefer to make that stuff robust than having crystal ball
constants and magic debugfs knobs.

Thanks,

	tglx