linux-kernel - Re: Warn people about flush_scheduled

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Date:	Tue, 15 Dec 2009 07:38:31 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Warn people about flush_scheduled_work()

Hello, Alan Stern.

On 12/15/2009 06:33 AM, Alan Stern wrote:
> You've spent some time working on the workqueue implementation, right?  
> I'd like to add comments or kerneldoc warning people about how 
> dangerous it can be to use flush_scheduled_work() and related 
> functions.  Something like this:
> 
> 	Think twice before calling this function!  It's very easy
> 	to get into trouble if you don't take great care.  Either
> 	of the following situations will lead to deadlock:
> 
> 		Your code is running in the context of a scheduled
> 		work routine.
>
> 		Your code or its caller holds a lock needed by
> 		one of the work items currently on the workqueue.
>
> 	Since you generally don't know who your caller is, what locks
> 	it holds, or what locks are needed by the items on the 
> 	workqueue, avoiding these situations is quite difficult.

I think both problems can be detected by lockdep, right?  So, they
aren't that difficult to detect.

> 	Consider using cancel_work_sync() or cancel_delayed_work_sync()
> 	instead.  In most situations they will accomplish what you 
> 	need.
> 
> Does this sound like a good idea?  Certainly flush_scheduled_work()  
> is used in places where it shouldn't be.

Yeah, recommending more work-specific constructs definitely would be
better.  It's bad that we can't recommend the use of flush_work() as
it doesn't do cross-cpu flushing.  Maybe that needs explanation too.

> If comments like this are added, where do you think would be a good 
> place to put them?

DocBook comment on top of each function, maybe?

Thanks.

-- 
tejun
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