Re: [PATCH] blk: fix dangling pointer access in __elv_add_request

[Tejun Heo <htejun@gmail.com>]

Jens Axboe wrote:
> On Tue, Nov 01 2005, Tejun Heo wrote:
>
> > cfq's add_req_fn callback may invoke q->request_fn directly and
> > depending on low-level driver used and timing, a queued request may be
> > finished & deallocated before add_req_fn callback returns.  So,
> > __elv_add_request must not access rq after it's passed to add_req_fn
> > callback.
>
>
> It's a generel problem, you may get the queue run at any time regardless
> of what the io scheduler is doing. CFQ does run the queue manully
> sometimes, but SCSI may do the very same thing for you as well. Given
> that SCSI also shortly reenables interrupts in the ->request_fn()
> handling, it's quite possible for the request to be completed.
>
> So, as we don't hold a reference to the request, I'd say your patch
> looks correct and should be applied right away.
>
>
>>Jens, does generalizing queue kicking functions and disallowing
>>ioscheds from directly calling q->request_fn sound like a good idea?
>
>
> Yes certainly.
>

The thing is that we are holding queue_lock before calling add_req_fn 
callback and also after it finishes giving it an appearance of 
atomicity.  I think q->request_fn semantics is peculiar and a bit prone 
to bug, so it might be better to make ioscheds always use generic queue 
kicking function which always uses work queue to run q->request_fn so 
that we don't have queue_lock releasing and regrabbing inbetween.  Do 
you think there can be any noticieable performance issues?

Hmmmm... One more thing about q->request_fn's locking behavior is that, 
as I noted while posting the ordered patchset, for SCSI, the behavior 
can reorder issued requests making it impossible to use ordered tags for 
flushing.  I'm thinking of submitting a patch to make scsi request_fn 
atomic w.r.t. queue_lock, but there might be some performance issues I'm 
not aware of.  Functions which release and regrab locks underneath the 
caller are just... hard.  :-p

Thanks.

--
tejun
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