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    Subject[patch] i915: take struct_mutex in i915_dma_cleanup()
    intel_cleanup_ring_buffer() calls drm_gem_object_unreference() (as
    opposed to drm_gem_object_unreference_unlocked()) so it needs to be
    called with "struct_mutex" held. If we don't hold the lock, it triggers
    a BUG_ON(!mutex_is_locked(&dev->struct_mutex));

    I also audited the other places that call intel_cleanup_ring_buffer()
    and they all hold the lock so they're OK.

    This was introduced in: 8187a2b70e3 "drm/i915: introduce
    intel_ring_buffer structure (V2)" and it's a regression from v2.6.34.

    Addresses: https://bugzilla.kernel.org/show_bug.cgi?id=16247

    Signed-off-by: Dan Carpenter <error27@gmail.com>
    Reported-by: Benny Halevy <bhalevy@panasas.com>
    Tested-by: Benny Halevy <bhalevy@panasas.com>

    diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
    index 59a2bf8..3325af1 100644
    --- a/drivers/gpu/drm/i915/i915_dma.c
    +++ b/drivers/gpu/drm/i915/i915_dma.c
    @@ -128,9 +128,11 @@ static int i915_dma_cleanup(struct drm_device * dev)
    if (dev->irq_enabled)
    drm_irq_uninstall(dev);

    + mutex_lock(&dev->struct_mutex);
    intel_cleanup_ring_buffer(dev, &dev_priv->render_ring);
    if (HAS_BSD(dev))
    intel_cleanup_ring_buffer(dev, &dev_priv->bsd_ring);
    + mutex_unlock(&dev->struct_mutex);

    /* Clear the HWS virtual address at teardown */
    if (I915_NEED_GFX_HWS(dev))


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