linux-kernel - Re: [PATCH 1/2] watchdog: introduce touch_softlockup_watchdog

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Date:	Thu, 3 Dec 2015 11:00:18 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Ulrich Obergfell <uobergfe@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 1/2] watchdog: introduce touch_softlockup_watchdog_sched()

On Thu, Dec 03, 2015 at 10:33:50AM +0100, Peter Zijlstra wrote:
> On Wed, Dec 02, 2015 at 07:28:10PM -0500, Tejun Heo wrote:
> > Hello,
> > 
> > There haven't been too many workqueue stall bugs; however, good part
> > of them have been pretty painful to track down because there's no
> > lockup detection mechanism for workqueue and it isn't easy to tell
> > what's going on with workqueues; furthermore, some requirements are
> > tricky to get right - e.g. it's not too difficult to miss
> > WQ_MEM_RECLAIM for a workqueue which runs a work item which is flushed
> > by something which sits in the reclaim path.
> 
> have you considered something as simple as:
> 
> 	WARN_ON(current->reclaim_state && !WQ_MEM_RECLAIM);
> 
> ?

Alternatively, you can 'abuse' the lockdep reclaim bits by marking
!MEM_RECLAIM workqueue 'locks' with lockdep_trace_alloc(GFP_KERNEL),
that way lockdep will yell if they get used in a reclaim context.

This might be a tad tricky in that you need 2 sets of (lockdep) keys for
things.
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