Messages in this thread | | | Date | Tue, 18 Feb 2014 11:31:18 -0500 | From | Peter Hurley <> | Subject | Re: [PATCH] workqueue: Document exceptions to work item non-reentrancy guarantee |
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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
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