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    SubjectRe: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request() to be called from interrupt context
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    Hi Jens,

    On Wed, 20 Dec 2006 19:49:17 +0100, Jens Axboe <jens.axboe@oracle.com> wrote:
    > > > Big NACK on this - it's not only really ugly, it's also buggy to pass
    > > > interrupt flags as function arguments. As you also mention in the 0/1
    > > > mail, this also breaks CFQ.
    > > >
    > > > Why do you need in-interrupt request allocation?
    > >
    > > Because I'd like to use blk_get_request() in q->request_fn()
    > > which can be called from interrupt context like below:
    > > scsi_io_completion -> scsi_end_request -> scsi_next_command
    > > -> scsi_run_queue -> blk_run_queue -> q->request_fn
    > >
    > > Generally, device-mapper (dm) clones an original I/O and dispatches
    > > the clones to underlying destination devices.
    > > In the request-based dm patch, the clone creation and the dispatch
    > > are done in q->request_fn(). To create the clone, blk_get_request()
    > > is used to get a request from underlying destination device's queue.
    > > By doing that in q->request_fn(), dm can deal with struct request
    > > after bios are merged by __make_request().
    > >
    > > Do you think creating another function like blk_get_request_nowait()
    > > is acceptable?
    > > Or request should not be allocated in q->request_fn() anyway?
    >
    > You should not be allocating requests from that path, for a number of
    > reasons.

    Could I hear the reasons for my further work if possible?
    Because of breaking current CFQ? And is there any reason?


    > The design isn't very nice either.
    >
    > The easy way out would be to punt to a workqueue to handle the requests.
    >
    > An alternative way would be to set aside some requests that you can get
    > at without allocation (maintain a little freelist of manually allocated
    > requests), and retrieve a free one from there when inside request_fn. If
    > you run out, just bail out of request_fn and make sure to reinvoke it
    > when some of your previously issued requests complete and are added back
    > to that freelist.

    Thank you for the suggestions.
    OK, I'll think other designs based on your suggestions.

    Thanks,
    Kiyoshi Ueda

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