Re: [PATCH v4 1/2] SCSI: Asynchronous event notification infrastructure

[Jeff Garzik <jeff@garzik.org>]

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 :)


> 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...

	Jeff


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