From mboxrd@z Thu Jan  1 00:00:00 1970
From: Claude Code Review Bot <claude-review@example.com>
To: dri-devel-reviews@example.com
Subject: Claude review: drm/ttm/tests: Fix build failure on PREEMPT_RT
Date: Thu, 05 Mar 2026 13:40:45 +1000
Message-ID: <review-patch1-20260304085616.1216961-1-dev@lankhorst.se>
In-Reply-To: <20260304085616.1216961-1-dev@lankhorst.se>
References: <20260304085616.1216961-1-dev@lankhorst.se> <20260304085616.1216961-1-dev@lankhorst.se>
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Patch Review

**Problem:** The test `ttm_bo_reserve_interrupted` directly locks the under=
lying mutex of the dma_resv's ww_mutex via `mutex_lock(&bo->base.resv->lock=
.base)`. On PREEMPT_RT, `struct mutex` becomes `struct rt_mutex` with a dif=
ferent layout, causing a compile error.

**Fix analysis:**

The change from:
```c
mutex_lock(&bo->base.resv->lock.base);
...
mutex_unlock(&bo->base.resv->lock.base);
```
to:
```c
dma_resv_lock(bo->base.resv, NULL);
...
dma_resv_unlock(bo->base.resv);
```

is correct. Looking at the implementation in `include/linux/dma-resv.h:342`=
, `dma_resv_lock()` with a NULL context calls `ww_mutex_lock(&obj->lock, NU=
LL)`, which behaves as a plain lock without deadlock avoidance =E2=80=94 se=
mantically equivalent to what the old code was doing by locking the base mu=
tex directly.

**Semantic correctness:** The test's purpose is for the main thread to hold=
 the reservation lock, then have `threaded_ttm_bo_reserve` (line 184) attem=
pt to acquire it via `ttm_bo_reserve()` with a proper `ww_acquire_ctx`, exp=
ecting the thread to be interrupted and return `-ERESTARTSYS`. Using `dma_r=
esv_lock(NULL)` vs raw `mutex_lock` on the base doesn't change the contenti=
on behavior =E2=80=94 the threaded locker will still block and get signaled.

**Minor note:** `dma_resv_unlock()` (line 461) calls `dma_resv_reset_max_fe=
nces(obj)` before `ww_mutex_unlock()`, which zeroes `obj->max_fences`. This=
 is a harmless side effect in this test context since no fences are attache=
d.

**v1 =E2=86=92 v2 change:** The changelog says "Use the correct locking pri=
mitive (Jouni)", which aligns with this being the v2 improvement =E2=80=94 =
using the proper API rather than some other workaround (likely `#ifdef PREE=
MPT_RT` or similar in v1).

**Reviewed-by worthy.** No issues found.

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