linux-kernel - Re: [PATCH] scheduler: convert BUG_ON()s in try_to_wake_up_local() to WARN_ON

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Date:	Tue, 19 Mar 2013 07:29:54 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scheduler: convert BUG_ON()s in try_to_wake_up_local()
 to WARN_ON_ONCE()s

On Tue, 2013-03-19 at 11:14 +0100, Peter Zijlstra wrote:
> On Mon, 2013-03-18 at 12:22 -0700, Tejun Heo wrote:
> > try_to_wake_up_local() should only be invoked to wake up another task
> > in the same runqueue and BUG_ON()s are used to enforce the rule.
> > Missing try_to_wake_up_local() can stall workqueue execution but such
> > stalls are likely to be finite either by another work item being
> > queued or the one blocked getting unblocked.  There's no reason to
> > trigger BUG while holding rq lock crashing the whole system.
> > 
> > Convert BUG_ON()s in try_to_wake_up_local() to WARN_ON_ONCE()s.
> 
> Doesn't really matter either way, the printk()s triggered by either
> will very likely kill the system anyhow ;-)

Only if something blocks on the console lock, and it tries to wake it
up. But even then, it has to be a reverse rq locking order, or wakeup on
the same rq that its running on (which is very unlikely).

I admit that it doesn't "solve" the bug. But at least there's a chance
to investigate what happened when the bug does occur. Right now we have
only one conclusion when the bug triggers, and that's a hard lockup.

-- Steve


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