[Bug 92480] [Patch] Linux mutex leak in drm_dp_get_mst_branch

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92480

            Bug ID: 92480
           Summary: [Patch] Linux mutex leak in drm_dp_get_mst_branch
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: General
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: adam_richter2004 at yahoo.com

I believe I have found a mutex leak in drm_dp_get_mst_branch_device() in
linux/drivers/gpu/drm/drm_dp_mst_topology.c in kernels 4.2-rc1 through the
current version, 4.3-rc5, where the function can take an error exit without
doing mutex_unlock(&mgr_lock).  I have attached a proposed patch.

Unfortunately, since I made the patch, I have not been able to trigger the
error branch that the patch fixes, but I did find the bug by observing that
error branch executing and printing an error message that I believe is unique
to that branch ("*ERROR* failed to lookup MSTB"):

[   36.944273] [drm:gen8_irq_handler [i915]] *ERROR* The master control
interrupt lied (SDE)!
[   74.576811] [drm:drm_dp_get_mst_branch_device [drm_kms_helper]] *ERROR*
failed to lookup MSTB with lct 2, rad 10
[   74.587704] BUG: workqueue leaked lock or atomic: kworker/u8:0/0x00000000/6
[   74.587704]      last function: i915_digport_work_func [i915]
[   74.655752] [drm:gen8_irq_handler [i915]] *ERROR* The master control
interrupt lied (SDE)!

I think this patch is also arguably an infinitesimal improvement in that it
depends less on the optimizer, by reducing two references to port->mstb to one.

Anyhow, if the patch looks good, I would be delighted to if the appropriate
person (Dave?) would integrate it for submission upstream.

Thanks in advance for any feedback on this.

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