[PATCH v2 1/3] drm/msm: Protect obj->active_count under obj lock

Rob Clark
2020-11-16T17:47:57Z

Previously we only held obj lock in the _active_get() path, and relied
on atomic_dec_return() to not be racy in the _active_put() path where
obj lock was not held.

But this is a false sense of security. Unlike obj lifetime refcnt,
where you do not expect to *increase* the refcnt after the last put
(which would mean that something has gone horribly wrong with the
object liveness reference counting), the active_count can increase
again from zero. Racing _active_put()s and _active_get()s could leave
the obj on the wrong mm list.

But in the retire path, immediately after the _active_put(), the
_unpin_iova() would acquire obj lock. So just move the locking earlier
and rely on that to protect obj->active_count.

Fixes: c5c1643cef7a ("drm/msm: Drop struct_mutex from the retire path")
Signed-off-by: Rob Clark <robdclark@chromium.org>
---
drivers/gpu/drm/msm/msm_gem.c | 11 ++++++-----
drivers/gpu/drm/msm/msm_gem.h | 5 +++--
drivers/gpu/drm/msm/msm_gpu.c | 10 ++++++----
3 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
index a9a834bb7794..2795288b0a95 100644
--- a/drivers/gpu/drm/msm/msm_gem.c
+++ b/drivers/gpu/drm/msm/msm_gem.c
@@ -770,7 +770,7 @@ void msm_gem_active_get(struct drm_gem_object *obj, struct msm_gpu *gpu)
WARN_ON(!msm_gem_is_locked(obj));
WARN_ON(msm_obj->madv != MSM_MADV_WILLNEED);

- if (!atomic_fetch_inc(&msm_obj->active_count)) {
+ if (msm_obj->active_count++ == 0) {
mutex_lock(&priv->mm_lock);
list_del_init(&msm_obj->mm_list);
list_add_tail(&msm_obj->mm_list, &gpu->active_list);
@@ -784,8 +784,9 @@ void msm_gem_active_put(struct drm_gem_object *obj)
struct msm_drm_private *priv = obj->dev->dev_private;

might_sleep();
+ WARN_ON(!msm_gem_is_locked(obj));

- if (!atomic_dec_return(&msm_obj->active_count)) {
+ if (--msm_obj->active_count == 0) {
mutex_lock(&priv->mm_lock);
list_del_init(&msm_obj->mm_list);
list_add_tail(&msm_obj->mm_list, &priv->inactive_list);
@@ -936,15 +937,15 @@ void msm_gem_free_object(struct drm_gem_object *obj)
struct drm_device *dev = obj->dev;
struct msm_drm_private *priv = dev->dev_private;

- /* object should not be on active list: */
- WARN_ON(is_active(msm_obj));
-
mutex_lock(&priv->mm_lock);
list_del(&msm_obj->mm_list);
mutex_unlock(&priv->mm_lock);

msm_gem_lock(obj);

+ /* object should not be on active list: */
+ WARN_ON(is_active(msm_obj));
+
put_iova(obj);

if (obj->import_attach) {
diff --git a/drivers/gpu/drm/msm/msm_gem.h b/drivers/gpu/drm/msm/msm_gem.h
index d79e7019cc88..3355a48a023b 100644
--- a/drivers/gpu/drm/msm/msm_gem.h
+++ b/drivers/gpu/drm/msm/msm_gem.h
@@ -87,7 +87,7 @@ struct msm_gem_object {

char name[32]; /* Identifier to print for the debugfs files */

- atomic_t active_count;
+ int active_count;
};
#define to_msm_bo(x) container_of(x, struct msm_gem_object, base)

@@ -185,7 +185,8 @@ msm_gem_is_locked(struct drm_gem_object *obj)

static inline bool is_active(struct msm_gem_object *msm_obj)
{
- return atomic_read(&msm_obj->active_count);
+ WARN_ON(!msm_gem_is_locked(&msm_obj->base));
+ return msm_obj->active_count;
}

static inline bool is_purgeable(struct msm_gem_object *msm_obj)
diff --git a/drivers/gpu/drm/msm/msm_gpu.c b/drivers/gpu/drm/msm/msm_gpu.c
index b597213a1890..04f7ab4d63ae 100644
--- a/drivers/gpu/drm/msm/msm_gpu.c
+++ b/drivers/gpu/drm/msm/msm_gpu.c
@@ -717,11 +717,13 @@ static void retire_submit(struct msm_gpu *gpu, struct msm_ringbuffer *ring,
stats->alwayson_start, stats->alwayson_end);

for (i = 0; i < submit->nr_bos; i++) {
- struct msm_gem_object *msm_obj = submit->bos[i].obj;
+ struct drm_gem_object *obj = &submit->bos[i].obj->base;

- msm_gem_active_put(&msm_obj->base);
- msm_gem_unpin_iova(&msm_obj->base, submit->aspace);
- drm_gem_object_put(&msm_obj->base);
+ msm_gem_lock(obj);
+ msm_gem_active_put(obj);
+ msm_gem_unpin_iova_locked(obj, submit->aspace);
+ msm_gem_unlock(obj);
+ drm_gem_object_put(obj);
}

pm_runtime_mark_last_busy(&gpu->pdev->dev);
--
2.28.0