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

[Tejun Heo <tj@kernel.org>]

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