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    SubjectRe: [PATCH v3 2/9] drm: hold master_lookup_lock when releasing a drm_file's master
    On Wed, Aug 18, 2021 at 03:38:17PM +0800, Desmond Cheong Zhi Xi wrote:
    > When drm_file.master changes value, the corresponding
    > drm_device.master_lookup_lock should be held.
    >
    > In drm_master_release, a call to drm_master_put sets the
    > file_priv->master to NULL, so we protect this section with
    > drm_device.master_lookup_lock.
    >
    > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

    At this points all refcounts to drm_file have disappeared, so yeah this is
    a lockless access, but also no one can observe it anymore. See also next
    patch.

    Hence I think the current code is fine.
    -Daniel

    > ---
    > drivers/gpu/drm/drm_auth.c | 5 ++++-
    > 1 file changed, 4 insertions(+), 1 deletion(-)
    >
    > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
    > index 8efb58aa7d95..8c0e0dba1611 100644
    > --- a/drivers/gpu/drm/drm_auth.c
    > +++ b/drivers/gpu/drm/drm_auth.c
    > @@ -373,8 +373,11 @@ void drm_master_release(struct drm_file *file_priv)
    > }
    >
    > /* drop the master reference held by the file priv */
    > - if (file_priv->master)
    > + if (file_priv->master) {
    > + spin_lock(&dev->master_lookup_lock);
    > drm_master_put(&file_priv->master);
    > + spin_unlock(&dev->master_lookup_lock);
    > + }
    > mutex_unlock(&dev->master_mutex);
    > }
    >
    > --
    > 2.25.1
    >

    --
    Daniel Vetter
    Software Engineer, Intel Corporation
    http://blog.ffwll.ch

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