Re: [NBD] Use per-device semaphore instead of BKL

From: Paul Clements (paul.clements_at_steeleye.com)
Date: 11/20/05

  • Next message: Andi Kleen: "(no subject)" 
    Date:	Sun, 20 Nov 2005 12:19:17 -0500
To: Herbert Xu <herbert@gondor.apana.org.au>

    Herbert Xu wrote:
    > On Sun, Nov 20, 2005 at 09:34:19AM +1100, herbert wrote:
    >
    >>This is intentional actually. nbd_clear_queue never races against
    >>nbd_find_request because the ioctl is protected by the BKL. If it
    >>weren't, then we have much worse problems to worry about (e.g.,
    >>while you're clearing the queue someone else could have set the
    >>socket again and started queueing requests).
    >
    >
    > Actually, we do have bigger problems :) The BKL is dropped every
    > time you sleep, and nbd_do_it is definitely a frequent sleeper :)

    The dropping of the lock in nbd_do_it is actually critical to the way
    nbd functions. nbd_do_it runs for the lifetime of the nbd device, so if
    nbd_do_it were holding some lock (BKL or otherwise), we'd have big problems.

    > This isn't really an issue in practice though because the NBD
    > client program is single-threaded and doesn't share its file
    > descriptors with anyone else.

    Right, there's no problem in practice.

    > However, we shouldn't make it too easy for the user to shoot himself
    > in the foot. If he's going to do that, let him at least pay for the
    > bullet :)
    >
    > So here is a patch to use a per-device semaphore instead of the
    > BKL to protect the ioctl's against each other.

    The problem with this patch is that no ioctls can come in once nbd_do_it
    starts because nbd_do_it runs for the lifetime of the device.

    I think we really just need to add the acquiring of queue_lock in
    nbd_clear_que to your previous patch and leave it at that. I'll code
    that up and test it.

    Thanks,
    Paul
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