Re: 4.9-rc1 lockdep warning suggesting a deadlock between nouveau and i915 with prime video outputs active

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Jiri Slaby <jirislaby@gmail.com>, Hans de Goede <hdegoede@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Ben Skeggs <bskeggs@redhat.com>,
	dri-devel@lists.freedesktop.org
Subject: Re: 4.9-rc1 lockdep warning suggesting a deadlock between nouveau and i915 with prime video outputs active
Date: Thu, 13 Jul 2017 15:04:58 +0100	[thread overview]
Message-ID: <149995469805.29380.5405466287939095053@mail.alporthouse.com> (raw)
In-Reply-To: <2007195d-6663-867a-ecc3-03d2d348fb72@gmail.com>

Quoting Jiri Slaby (2017-07-13 14:57:31)
> Stealing this thread as an opensuse user hit that too:
> https://bugzilla.suse.com/show_bug.cgi?id=1045105

It's a false positive. I did once upon a time send some patches to move
the lockdep warning to kref so that didn't need to call it from drm
before an unlocked path. Basically you want

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 8dc11064253d..3118aed844f1 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -826,7 +826,6 @@ drm_gem_object_put_unlocked(struct drm_gem_object *obj)
                return;
 
        dev = obj->dev;
-       might_lock(&dev->struct_mutex);
 
        if (dev->driver->gem_free_object_unlocked)
                kref_put(&obj->refcount, drm_gem_object_free);
diff --git a/include/linux/kref.h b/include/linux/kref.h
index 29220724bf1c..4b1133cd5d20 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -77,6 +77,8 @@ static inline int kref_put_mutex(struct kref *kref,
                                 void (*release)(struct kref *kref),
                                 struct mutex *lock)
 {
+       might_lock(lock);
+
        if (refcount_dec_and_mutex_lock(&kref->refcount, lock)) {
                release(kref);
                return 1;


Though now we probably want to move that might_lock() into refcount.
-Chris
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