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

[Peter Hurley <peter@hurleysoftware.com>]

On 02/18/2014 10:30 AM, Tejun Heo wrote:
> 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.

Ok.

That doesn't make the use-case go away; it simply moves it outside
the workqueue subsystem.

For example, in the case of the firewire subsystem, this technique
was used to essentially single-thread per-device work using only one
designated workqueue for all devices. The possibility of accidentally
running a work item 2x is a non-issue since the device state is
managed atomically.

Of the other use cases in the kernel, it seems only the floppy
driver uses a similar technique. But maybe that's ok because it's
on a single-threaded workqueue.

USB and AFS use PREPARE_{DELAYED}_WORK to reschedule from within
the current work function to a new function, which seems ok.

fwserial already serializes its use of PREPARE_WORK with &peer->lock
(and checks if the work is already pending).

> It's a pretty dumb thing to do anyway.

Fragile, yes; dumb, no. At least not from the point-of-view of the
documentation and what the workqueue actually did. But obviously from
your reaction, unintentional design.

> I'll look into it.

Thanks.

Regards,
Peter Hurley