[PATCH v2] drm/i915: Fix ref->mutex deadlock in i915_active_wait()

[Sultan Alsawaf]

On Fri, Apr 10, 2020 at 11:08:38AM +0200, Greg KH wrote:
> On Tue, Apr 07, 2020 at 12:18:09AM -0700, Sultan Alsawaf wrote:
> > From: Sultan Alsawaf <sultan at kerneltoast.com>
> > 
> > The following deadlock exists in i915_active_wait() due to a double lock
> > on ref->mutex (call chain listed in order from top to bottom):
> >  i915_active_wait();
> >  mutex_lock_interruptible(&ref->mutex); <-- ref->mutex first acquired
> >  i915_active_request_retire();
> >  node_retire();
> >  active_retire();
> >  mutex_lock_nested(&ref->mutex, SINGLE_DEPTH_NESTING); <-- DEADLOCK
> > 
> > Fix the deadlock by skipping the second ref->mutex lock when
> > active_retire() is called through i915_active_request_retire().
> > 
> > Note that this bug only affects 5.4 and has since been fixed in 5.5.
> > Normally, a backport of the fix from 5.5 would be in order, but the
> > patch set that fixes this deadlock involves massive changes that are
> > neither feasible nor desirable for backporting [1][2][3]. Therefore,
> > this small patch was made to address the deadlock specifically for 5.4.
> > 
> > [1] 274cbf20fd10 ("drm/i915: Push the i915_active.retire into a worker")
> > [2] 093b92287363 ("drm/i915: Split i915_active.mutex into an irq-safe spinlock for the rbtree")
> > [3] 750bde2fd4ff ("drm/i915: Serialise with remote retirement")
> > 
> > Fixes: 12c255b5dad1 ("drm/i915: Provide an i915_active.acquire callback")
> > Cc: <stable at vger.kernel.org> # 5.4.x
> > Signed-off-by: Sultan Alsawaf <sultan at kerneltoast.com>
> > ---
> >  drivers/gpu/drm/i915/i915_active.c | 27 +++++++++++++++++++++++----
> >  drivers/gpu/drm/i915/i915_active.h |  4 ++--
> >  2 files changed, 25 insertions(+), 6 deletions(-)
> 
> Now queued up, thanks.
> 
> greg k-h

I'm sorry, I meant the v3 [1]. Apologies for the confusion. The v3 was picked
into Ubuntu so that's what we're rolling with.

Thanks,
Sultan

[1] https://lore.kernel.org/stable/20200407203222.2493-1-sultan@kerneltoast.com