On Mon, 2007-10-29 at 12:29 -0400, Jeff Garzik wrote:
> James Bottomley wrote:
> > On Mon, 2007-10-29 at 12:07 -0400, Jeff Garzik wrote:
> >> James Bottomley wrote:
> >>> This still doesn't solve the fundamental corruption problem:
> >>> sdev->event_work has to contain the work entry until the workqueue has
> >>> finished executing it (which is some unspecified time in the future).
> >>> As soon as you drop the sdev->list_lock, the system thinks
> >>> sdev->event_work is available for reuse. If we fire another event
> >>> before the work queue finished processing the prior event, the queue
> >>> will be corrupted.
> >> I think you're misunderstanding the workqueue code? You can call
> >> schedule_work(&sdev->event_work) from anywhere, any time you like, as
> >> many times as you like.
> >
> > OK, take me through it slowly then ... I think schedule_work(work)
> > inserts work->entry onto the workqueue list (in
> > workqueue.c:insert_work()). If the event hasn't fired, it will already
> > be on the list, so adding the same entry to a list twice causes a list
> > corruption problem.
>
> It does a test_and_set_bit() first thing in queue_work(). Similar
> exclusivity logic is found in net device land. Ah, the fun of locking
> without locks that benh grumbles about :)
Ah, OK, sorry ... I was actually looking at __queue_work().
> > Plus, unfortunately, the CC/UA events are going to have to carry extra
> > sense data; they're not simply going to be triggers saying something
> > happened.
>
> OK this is a fair criticism.
>
> If additional data must be carried, then I must ditch the beloved bitmap
> implementation and go back to a list (with associated GFP_ATOMIC alloc).
>
> I will fix this, unless I receive email to the contrary...
Yes, unfortunately, thanks. If all events were a simple number, it's
easy, but the CC/UA events carry data as well.
James
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