[Intel-xe] [PATCH 2/3] drm/xe: fix xe_device_mem_access_get() race

[Matthew Auld]

It looks like there is at least one race here, given that the
pm_runtime_suspended() check looks to return false if we are in the
process of suspending the device (RPM_SUSPENDING vs RPM_SUSPENDED). We
later also do xe_pm_runtime_get_if_active(), but since the device is
suspending or has now suspended, this doesn't do anything either.
Following from this we can potentially return from
xe_device_mem_access_get() with the device suspended or about to be,
leading to broken behaviour.

Attempt to fix this by always grabbing the runtime ref when our internal
ref transitions from 0 -> 1, and then wrap the whole thing with a lock
to ensure callers are serialized.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/258
Signed-off-by: Matthew Auld <matthew.auld at intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Cc: Matthew Brost <matthew.brost at intel.com>
---
 drivers/gpu/drm/xe/xe_device.c       | 11 +++--------
 drivers/gpu/drm/xe/xe_device_types.h |  5 ++++-
 drivers/gpu/drm/xe/xe_pm.c           |  9 ++-------
 drivers/gpu/drm/xe/xe_pm.h           |  2 +-
 4 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_device.c b/drivers/gpu/drm/xe/xe_device.c
index 01c497bcf9a5..0a18b41a0e1a 100644
--- a/drivers/gpu/drm/xe/xe_device.c
+++ b/drivers/gpu/drm/xe/xe_device.c
@@ -406,17 +406,12 @@ u32 xe_device_ccs_bytes(struct xe_device *xe, u64 size)
 
 void xe_device_mem_access_get(struct xe_device *xe)
 {
-	bool resumed = xe_pm_runtime_resume_if_suspended(xe);
-
 	mutex_lock(&xe->mem_access.lock);
-	if (xe->mem_access.ref++ == 0)
-		xe->mem_access.hold_rpm = xe_pm_runtime_get_if_active(xe);
+	if (xe->mem_access.ref == 0)
+		xe->mem_access.hold_rpm = xe_pm_runtime_resume_and_get(xe);
+	xe->mem_access.ref++;
 	mutex_unlock(&xe->mem_access.lock);
 
-	/* The usage counter increased if device was immediately resumed */
-	if (resumed)
-		xe_pm_runtime_put(xe);
-
 	XE_WARN_ON(xe->mem_access.ref == S32_MAX);
 }
 
diff --git a/drivers/gpu/drm/xe/xe_device_types.h b/drivers/gpu/drm/xe/xe_device_types.h
index 59462933f67a..9e37189d5745 100644
--- a/drivers/gpu/drm/xe/xe_device_types.h
+++ b/drivers/gpu/drm/xe/xe_device_types.h
@@ -256,7 +256,10 @@ struct xe_device {
 	 * triggering additional actions when they occur.
 	 */
 	struct {
-		/** @lock: protect the ref count */
+		/**
+		 * @lock: Serialize xe_device_mem_access users,
+		 * and protect the below internal state, like @ref.
+		 */
 		struct mutex lock;
 		/** @ref: ref count of memory accesses */
 		s32 ref;
diff --git a/drivers/gpu/drm/xe/xe_pm.c b/drivers/gpu/drm/xe/xe_pm.c
index b7b57f10ba25..b2ffa001e6f7 100644
--- a/drivers/gpu/drm/xe/xe_pm.c
+++ b/drivers/gpu/drm/xe/xe_pm.c
@@ -210,14 +210,9 @@ int xe_pm_runtime_put(struct xe_device *xe)
 	return pm_runtime_put_autosuspend(xe->drm.dev);
 }
 
-/* Return true if resume operation happened and usage count was increased */
-bool xe_pm_runtime_resume_if_suspended(struct xe_device *xe)
+bool xe_pm_runtime_resume_and_get(struct xe_device *xe)
 {
-	/* In case we are suspended we need to immediately wake up */
-	if (pm_runtime_suspended(xe->drm.dev))
-		return !pm_runtime_resume_and_get(xe->drm.dev);
-
-	return false;
+	return !pm_runtime_resume_and_get(xe->drm.dev);
 }
 
 int xe_pm_runtime_get_if_active(struct xe_device *xe)
diff --git a/drivers/gpu/drm/xe/xe_pm.h b/drivers/gpu/drm/xe/xe_pm.h
index 6a885585f653..1b4c15b5e71a 100644
--- a/drivers/gpu/drm/xe/xe_pm.h
+++ b/drivers/gpu/drm/xe/xe_pm.h
@@ -19,7 +19,7 @@ int xe_pm_runtime_suspend(struct xe_device *xe);
 int xe_pm_runtime_resume(struct xe_device *xe);
 int xe_pm_runtime_get(struct xe_device *xe);
 int xe_pm_runtime_put(struct xe_device *xe);
-bool xe_pm_runtime_resume_if_suspended(struct xe_device *xe);
+bool xe_pm_runtime_resume_and_get(struct xe_device *xe);
 int xe_pm_runtime_get_if_active(struct xe_device *xe);
 
 #endif
-- 
2.40.0