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

[Greg KH]

On Tue, Apr 14, 2020 at 09:15:07AM +0100, Chris Wilson wrote:
> Quoting Greg KH (2020-04-11 12:39:57)
> > On Fri, Apr 10, 2020 at 07:17:38AM -0700, Sultan Alsawaf wrote:
> > > 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.
> > 
> > Ok, thanks, hopefully now I picked upthe right one...
> 
> The patch does not fix a deadlock. Greg, this patch is not a backport of
> a bugfix, why is it in stable?

Because it says it can't be a backport as the 3 above mentioned patches
do the same thing instead?

I will be glad to drop this, but I need some kind of direction here, and
given that at least one distro is already shipping this, it felt like
the correct thing to do.

So, what do you want me to do?

thanks,

greg k-h