linux-kernel - Re: [PATCH] workqueue: Document exceptions to work item non-reentrancy guarantee

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Date:	Tue, 18 Feb 2014 10:30:56 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Ben Hutchings <ben@...adent.org.uk>, linux-kernel@...r.kernel.org,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	stable@...r.kernel.org
Subject: Re: [PATCH] workqueue: Document exceptions to work item
 non-reentrancy guarantee

On Mon, Feb 17, 2014 at 09:29:34PM -0500, Peter Hurley wrote:
> >It never would have occurred to me that you could safely change the
> >function for a work item that is already scheduled or running.
> >Especially given that PREPARE_WORK() is just a simple assignment (i.e.
> >no serialisation).
> 
> process_one_work() has an established order that safely allows for
> resetting the work function and scheduling the work, and further
> guaranteeing that the new work function will run.
> 
> Further, existing memory barriers ensure that
> 1. The new work function is visible on all cpus before testing if
>    the work is already pending.
> 2. The new work function is stored as the worker's current function
>    before the work is marked as not pending.
> 
> If this wasn't possible, then single-threaded workqueues could
> not be used for multiple functions without flushing work.
> 
> I wonder if the floppy driver is broken too.

Ugh... I'd just rather remove PREPARE_WORK altogether.  It's a pretty
dumb thing to do anyway.  I'll look into it.

Thanks.

-- 
tejun
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