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    SubjectRe: Driver crash on kernel 2.6.32.7. Interaction between cx8800 (DVB-S) and USB HVR Hauppauge 950q
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    On Thu, 2010-02-11 at 20:11 -0800, Greg KH wrote:
    > On Wed, Feb 10, 2010 at 10:34:56AM -0500, Andy Walls wrote:
    > > On Tue, 2010-02-09 at 22:12 -0500, Devin Heitmueller wrote:
    > > > On Tue, Feb 9, 2010 at 10:05 PM, Richard Lemieux <rlemieu@cooptel.qc.ca> wrote:
    > > > > Andy,
    > > > >
    > > > > This is a great answer! Thanks very much. When I get into this situation
    > > > > again
    > > > > I will know what to look for.
    > > > >
    > > > > A possible reason why I got into this problem in the first place is that I
    > > > > tried
    > > > > many combinations of parameters with mplayer and azap in order to learn how
    > > > > to use the USB tuner in both the ATSC and the NTSC mode. I will look back
    > > > > in the terminal history to see if I can find anything.
    > > >
    > > > I think the key to figuring out the bug at this point is you finding a
    > > > sequence where you can reliably reproduce the oops. If we have that,
    > > > then I can start giving you some code to try which we can see if it
    > > > addresses the problem.
    > > >
    > > > For example, I would start by giving you a fix which results in us not
    > > > calling the firmware release if the request_firmware() call failed,
    > > > but it wouldn't be much help if you could not definitively tell me if
    > > > the problem is fixed.
    > >
    > >
    > > For the oops analysis here:
    > >
    > > http://article.gmane.org/gmane.linux.drivers.video-input-infrastructure/15954
    > >
    > >
    > > I will also note that the file scope "fw_lock" mutex is rather
    > > inconsistently used in
    > > linux/drivers/base/fw_class.c:firmware_loading_store(). (I guess for
    > > not wanting to consume the timeout interval with sleeping?)
    > >
    > > The mutex protects "case 1:", but all other cases appear to be only
    > > protected by atomic status bit checks that can fall through to
    > > fw_load_abort() which complete()'s the fw_priv->completion.
    > >
    > > Also not that in the _request_firmware() this sequence is the only place
    > > a once good "fw_priv->fw" pointer is set to NULL:
    > >
    > > mutex_lock(&fw_lock);
    > > if (!fw_priv->fw->size || test_bit(FW_STATUS_ABORT, &fw_priv->status)) {
    > > retval = -ENOENT;
    > > release_firmware(fw_priv->fw);
    > > *firmware_p = NULL;
    > > }
    > > fw_priv->fw = NULL; <--------------- The only place it is set to NULL
    > > mutex_unlock(&fw_lock);
    > >
    > >
    > > So if the timeout timer fires at nearly the same time as udev coming in
    > > and say "I'm done loading" without holding the mutex, one can run into
    > > the Ooops. Not only that, I think the above code can leak memory under
    > > some circumstances when the "if" clause is not satisfied.
    > >
    > > I think this really is a firmware_class.c issue. I think the "just
    > > right" firmware loading timeouts and the particular computer system
    > > responsiveness, make this Ooops possible. However, I'm amazed that a
    > > single person has tripped it more than once.
    > >
    > > Revising the locking in linux/drivers/base/firmware_class.c should fix
    > > the problem.
    > >
    > > I don't believe this comment in the code now:
    > >
    > > /* fw_lock could be moved to 'struct firmware_priv' but since it is just
    > > * guarding for corner cases a global lock should be OK */
    > > static DEFINE_MUTEX(fw_lock);
    > >
    > > struct firmware_priv {
    > > char *fw_id;
    > > ...
    > >
    > > And since "f_priv" is dynamically created and destroyed by
    > > request_firmware() I see no harm in
    > >
    > > 1. moving the mutex into struct firmware_priv
    > > 2. just always just grabbing an almost never contended mutex
    > > 3. getting rid of the file scope fw_lock.
    > >
    > > except grabbing a mutex() while the timeout timer is running during
    > > loading, means one *could* sleep for a while consuming the timeout
    > > interval.
    >
    > That sounds reasonable to me, care to make up a patch for this?

    Yes. But it will take me a while. I don't have a git tree, because I
    don't have high bandwidth internet yet. (The cable company's been
    delayed in laying cable to my home due to repeated snowstorms.)

    I just didn't want the problem to fall through the cracks. I'll submit
    something to bugzilla for now. If a user complains of this rare Ooops
    when loading firmware, the current workaround is to lengthen the timeout
    via sysfs.

    Regards,
    Andy

    > thanks,
    >
    > greg k-h




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