[PATCH AUTOSEL 6.19-6.12] drm/ttm/tests: Fix build failure on PREEMPT_RT

[Sasha Levin]

From: Maarten Lankhorst <dev@xxxxxxxxxxxx>

[ Upstream commit a58d487fb1a52579d3c37544ea371da78ed70c45 ]

Fix a compile error in the kunit tests when CONFIG_PREEMPT_RT is
enabled, and the normal mutex is converted into a rtmutex.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202602261547.3bM6yVAS-lkp@xxxxxxxxx/
Reviewed-by: Jouni Högander <jouni.hogander@xxxxxxxxx>
Link: https://patch.msgid.link/20260304085616.1216961-1-dev@xxxxxxxxxxxx
Signed-off-by: Maarten Lankhorst <dev@xxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---

LLM Generated explanations, may be completely bogus:

## Analysis

**What the commit does:** Fixes a build failure in
`ttm_bo_reserve_interrupted()` kunit test when `CONFIG_PREEMPT_RT` is
enabled. On PREEMPT_RT, `struct mutex` becomes `struct rt_mutex`, so
direct `mutex_lock(&bo->base.resv->lock.base)` calls fail to compile.
The fix replaces the raw mutex calls with the proper
`dma_resv_lock()`/`dma_resv_unlock()` API, which correctly abstracts the
underlying lock type.

**Type of fix:** Build fix — prevents compilation failure under a valid
kernel configuration (PREEMPT_RT).

**Scope:** Extremely small — 2 lines changed in a test file. Changes
`mutex_lock` → `dma_resv_lock` and `mutex_unlock` → `dma_resv_unlock`.
The `dma_resv_*` API has been available for many kernel versions and is
the proper way to lock reservation objects.

**Affected versions:** The test file was introduced in v6.9-rc1 (commit
995279d280d1e). A similar fix for a *different function*
(`ttm_bo_reserve_deadlock`) was done in v6.11-rc1 (commit
f85376c890ef4), but it missed this second instance in
`ttm_bo_reserve_interrupted`. The buggy code exists in v6.12 and v6.13
stable trees. The file doesn't exist in v6.1 or v6.6 (pre-v6.9).

**Risk assessment:** Extremely low risk. This is a 2-line change in test
code only, using well-established APIs. It cannot cause runtime
regressions in production code. The `dma_resv_lock(obj, NULL)` call is
semantically equivalent to locking the underlying ww_mutex without a
context, which is what the original `mutex_lock` was doing.

**Stable criteria assessment:**
- Obviously correct and tested: Yes — uses standard API, reviewed by
  Jouni Högander, reported by kernel test robot
- Fixes a real bug: Yes — build failure with PREEMPT_RT
- Important issue: Build fixes are explicitly listed as stable-worthy in
  the rules
- Small and contained: Yes — 2 lines in one test file
- No new features: Correct

**Verification:**
- `git log --diff-filter=A` confirmed the test file was introduced in
  commit 995279d280d1e (v6.9-rc1)
- `git show v6.12:...ttm_bo_test.c` confirmed the buggy
  `mutex_lock(&bo->base.resv->lock.base)` exists at line 225 in v6.12
- `git show v6.13:...ttm_bo_test.c` confirmed the same buggy code exists
  in v6.13
- `git show v6.6/v6.1:...ttm_bo_test.c` confirmed the file does NOT
  exist in these older stable trees
- `git show v6.12:include/linux/dma-resv.h` confirmed
  `dma_resv_lock`/`dma_resv_unlock` API exists in v6.12
- Previous related fix f85376c890ef4 confirmed this is the same class of
  PREEMPT_RT build issue but in a different function
  (`ttm_bo_reserve_deadlock` vs `ttm_bo_reserve_interrupted`)
- The commit has `Reported-by: kernel test robot` and `Reviewed-by:
  Jouni Högander`, confirming it was properly tested and reviewed

This is a straightforward build fix — small, correct, zero runtime risk,
and applies to existing stable trees (v6.12, v6.13). Build fixes are
explicitly allowed in stable.

**YES**

 drivers/gpu/drm/ttm/tests/ttm_bo_test.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/ttm/tests/ttm_bo_test.c b/drivers/gpu/drm/ttm/tests/ttm_bo_test.c
index d468f83220720..f3103307b5df9 100644
--- a/drivers/gpu/drm/ttm/tests/ttm_bo_test.c
+++ b/drivers/gpu/drm/ttm/tests/ttm_bo_test.c
@@ -222,13 +222,13 @@ static void ttm_bo_reserve_interrupted(struct kunit *test)
 		KUNIT_FAIL(test, "Couldn't create ttm bo reserve task\n");
 
 	/* Take a lock so the threaded reserve has to wait */
-	mutex_lock(&bo->base.resv->lock.base);
+	dma_resv_lock(bo->base.resv, NULL);
 
 	wake_up_process(task);
 	msleep(20);
 	err = kthread_stop(task);
 
-	mutex_unlock(&bo->base.resv->lock.base);
+	dma_resv_unlock(bo->base.resv);
 
 	KUNIT_ASSERT_EQ(test, err, -ERESTARTSYS);
 }
-- 
2.51.0