Paulo Zanoni reported a lockdep splat with a locking inversion between
fpriv->fbs_lock and the modeset locks. This issue was introduced in
commit f2b50c1161590c3bcdbf3455fe4c575f1c1bd293
Author: Daniel Vetter <daniel.vetter@xxxxxxxx>
Date: Fri Sep 12 17:07:32 2014 +0200
drm: Fixup locking for universal cursor planes
This here is actually one of the rare cases where lockdep hits a false
positive: The deadlock only happens in drm_fb_release, which cleans up
the file private structure when all the references are gone. So the
locking is the very last one and no one else can deadlock. It also
doesn't protect anything at all, since all ioctls are guaranteed to
have returned at this point - otherwise they'd still hold a reference
on the file.
So let's just drop it and replace it with a big comment.
Cc: David Herrmann <dh.herrmann@xxxxxxxxx>
Cc: Matt Roper <matthew.d.roper@xxxxxxxxx>
Cc: Paulo Zanoni <przanoni@xxxxxxxxx>
Reported-by: Paulo Zanoni <przanoni@xxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
drivers/gpu/drm/drm_crtc.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index b7021069b078..e79c8d3700d8 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -3400,7 +3400,16 @@ void drm_fb_release(struct drm_file *priv)
struct drm_device *dev = priv->minor->dev;
struct drm_framebuffer *fb, *tfb;
- mutex_lock(&priv->fbs_lock);
+ /*
+ * When the file gets released that means no one else can access the fb
+ * list any more, so no need to grab fpriv->fbs_lock. And we need to to
+ * avoid upsetting lockdep since the universal cursor code adds a
+ * framebuffer while holding mutex locks.
+ *
+ * Note that a real deadlock between fpriv->fbs_lock and the modeset
+ * locks is impossible here since no one else but this function can get
+ * at it any more.
+ */
list_for_each_entry_safe(fb, tfb, &priv->fbs, filp_head) {
mutex_lock(&dev->mode_config.fb_lock);
@@ -3413,7 +3422,6 @@ void drm_fb_release(struct drm_file *priv)
/* This will also drop the fpriv->fbs reference. */
drm_framebuffer_remove(fb);
}
- mutex_unlock(&priv->fbs_lock);
}
/**
--
2.1.0
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