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    SubjectRe: [PATCH 3/3] [media] cx88: use a mutex to protect cx8802_devlist
    Hi Andreas,

    (please turn off HTML mail.)
    Andreas Huber wrote:

    > There is a reference count bug in the driver code. The driver's
    > active_ref count may become negative which leads to unpredictable
    > behavior. (mpeg video device inaccessible, etc ...)

    Hmm, the patchset didn't touch active_ref handling.

    active_ref was added by v2.6.25-rc3~132^2~7 (V4L/DVB (7194):
    cx88-mpeg: Allow concurrent access to cx88-mpeg devices, 2008-02-11)
    and relies on three assumptions:

    * (successful) calls to cx8802_driver::request_acquire are balanced
    with calls to cx8802_driver::request_release;

    * cx8802_driver::advise_acquire is non-null if and only if
    cx8802_driver::advise_release is (since both are NULL for
    blackbird, non-NULL for dvb);

    * no data races.

    I suppose it would be more idiomatic to use an atomic_t, but access to
    active_ref was previously protected by the BKL and now it is protected
    by core->lock. So it's not clear to me why this doesn't work.

    Any hints? (e.g., a detailed reproduction recipe, or a log after
    adding a printk to find out when exactly active_ref becomes negative)

    Thanks for reporting.

     \ /
      Last update: 2011-04-02 21:31    [W:0.020 / U:23.116 seconds]
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