linux-kernel - Re: [PATCH] bug: Set warn variable before calling WARN()

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Date:	Tue, 1 Mar 2016 17:35:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] bug: Set warn variable before calling WARN()

On Tue, Mar 01, 2016 at 11:09:39AM -0500, Steven Rostedt wrote:
> This has hit me a couple of times already. I would be debugging code
> and the system would simply hang and then reboot. Finally, I found that
> the problem was caused by WARN_ON_ONCE() and friends.
> 
> The macro WARN_ON_ONCE(condition) is defined as:
> 
> 	static bool __section(.data.unlikely) __warned;
> 	int __ret_warn_once = !!(condition);
> 
> 	if (unlikely(__ret_warn_once))
> 		if (WARN_ON(!__warned))
> 			__warned = true;
> 
> 	unlikely(__ret_warn_once);
> 
> Which looks great and all. But what I have hit, is an issue when
> WARN_ON() itself hits the same WARN_ON_ONCE() code. Because, the
> variable __warned is not yet set. Then it too calls WARN_ON() and that
> triggers the warning again. It keeps doing this until the stack is
> overflowed and the system crashes.
> 
> By setting __warned first before calling WARN_ON() makes the original
> WARN_ON_ONCE() really only warn once, and not an infinite amount of
> times if the WARN_ON() also triggers the warning.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>