[PATCH] kernel/locking: Add context to ww_mutex_trylock.

* [PATCH] kernel/locking: Add context to ww_mutex_trylock.
@ 2021-09-07 13:20 ` Maarten Lankhorst
  0 siblings, 0 replies; 54+ messages in thread
From: Maarten Lankhorst @ 2021-09-07 13:20 UTC (permalink / raw)
  To: intel-gfx
  Cc: dri-devel, Maarten Lankhorst, Peter Zijlstra, Ingo Molnar,
	Will Deacon, Waiman Long, Boqun Feng, Liam Girdwood, Mark Brown,
	linux-kernel

i915 will soon gain an eviction path that trylock a whole lot of locks
for eviction, getting dmesg failures like below:

BUG: MAX_LOCK_DEPTH too low!
turning off the locking correctness validator.
depth: 48  max: 48!
48 locks held by i915_selftest/5776:
 #0: ffff888101a79240 (&dev->mutex){....}-{3:3}, at: __driver_attach+0x88/0x160
 #1: ffffc900009778c0 (reservation_ww_class_acquire){+.+.}-{0:0}, at: i915_vma_pin.constprop.63+0x39/0x1b0 [i915]
 #2: ffff88800cf74de8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_vma_pin.constprop.63+0x5f/0x1b0 [i915]
 #3: ffff88810c7f9e38 (&vm->mutex/1){+.+.}-{3:3}, at: i915_vma_pin_ww+0x1c4/0x9d0 [i915]
 #4: ffff88810bad5768 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
 #5: ffff88810bad60e8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
...
 #46: ffff88811964d768 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
 #47: ffff88811964e0e8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
INFO: lockdep is turned off.

Fixing eviction to nest into ww_class_acquire is a high priority,
but it requires a rework of the entire driver, which can only be
done one step at a time.

As an intermediate solution, add an acquire context to ww_mutex_trylock,
which allows us to do proper nesting annotations on the trylocks, making
the above lockdep splat disappear.

This is also useful in regulator_lock_nested, which may avoid dropping
regulator_nesting_mutex in the uncontended path, so use it there.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 drivers/gpu/drm/drm_modeset_lock.c |  2 +-
 drivers/regulator/core.c           |  2 +-
 include/linux/dma-resv.h           |  2 +-
 include/linux/ww_mutex.h           | 13 +----
 kernel/locking/mutex.c             | 38 +++++++++++++
 kernel/locking/test-ww_mutex.c     | 86 ++++++++++++++++++++++--------
 lib/locking-selftest.c             |  2 +-
 7 files changed, 109 insertions(+), 36 deletions(-)

diff --git a/drivers/gpu/drm/drm_modeset_lock.c b/drivers/gpu/drm/drm_modeset_lock.c
index fcfe1a03c4a1..bf8a6e823a15 100644
--- a/drivers/gpu/drm/drm_modeset_lock.c
+++ b/drivers/gpu/drm/drm_modeset_lock.c
@@ -248,7 +248,7 @@ static inline int modeset_lock(struct drm_modeset_lock *lock,
 	if (ctx->trylock_only) {
 		lockdep_assert_held(&ctx->ww_ctx);
 
-		if (!ww_mutex_trylock(&lock->mutex))
+		if (!ww_mutex_trylock(&lock->mutex, NULL))
 			return -EBUSY;
 		else
 			return 0;
diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
index ca6caba8a191..f4d441b1a8bf 100644
--- a/drivers/regulator/core.c
+++ b/drivers/regulator/core.c
@@ -145,7 +145,7 @@ static inline int regulator_lock_nested(struct regulator_dev *rdev,
 
 	mutex_lock(&regulator_nesting_mutex);
 
-	if (ww_ctx || !ww_mutex_trylock(&rdev->mutex)) {
+	if (!ww_mutex_trylock(&rdev->mutex, ww_ctx)) {
 		if (rdev->mutex_owner == current)
 			rdev->ref_cnt++;
 		else
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index e1ca2080a1ff..39fefb86780b 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -173,7 +173,7 @@ static inline int dma_resv_lock_slow_interruptible(struct dma_resv *obj,
  */
 static inline bool __must_check dma_resv_trylock(struct dma_resv *obj)
 {
-	return ww_mutex_trylock(&obj->lock);
+	return ww_mutex_trylock(&obj->lock, NULL);
 }
 
 /**
diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
index b77f39f319ad..0b8f28577c00 100644
--- a/include/linux/ww_mutex.h
+++ b/include/linux/ww_mutex.h
@@ -313,17 +313,8 @@ ww_mutex_lock_slow_interruptible(struct ww_mutex *lock,
 
 extern void ww_mutex_unlock(struct ww_mutex *lock);
 
-/**
- * ww_mutex_trylock - tries to acquire the w/w mutex without acquire context
- * @lock: mutex to lock
- *
- * Trylocks a mutex without acquire context, so no deadlock detection is
- * possible. Returns 1 if the mutex has been acquired successfully, 0 otherwise.
- */
-static inline int __must_check ww_mutex_trylock(struct ww_mutex *lock)
-{
-	return mutex_trylock(&lock->base);
-}
+int __must_check ww_mutex_trylock(struct ww_mutex *lock,
+				  struct ww_acquire_ctx *ctx);
 
 /***
  * ww_mutex_destroy - mark a w/w mutex unusable
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index d2df5e68b503..5d0f5b04b568 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -1112,6 +1112,44 @@ __ww_mutex_lock(struct mutex *lock, unsigned int state, unsigned int subclass,
 	return __mutex_lock_common(lock, state, subclass, nest_lock, ip, ww_ctx, true);
 }
 
+/**
+ * ww_mutex_trylock - tries to acquire the w/w mutex with optional acquire context
+ * @lock: mutex to lock
+ * @ctx: optional w/w acquire context
+ *
+ * Trylocks a mutex with the optional acquire context; no deadlock detection is
+ * possible. Returns 1 if the mutex has been acquired successfully, 0 otherwise.
+ *
+ * Unlike ww_mutex_lock, no deadlock handling is performed. However, if a @ctx is
+ * specified, -EALREADY and -EDEADLK handling may happen in calls to ww_mutex_lock.
+ *
+ * A mutex acquired with this function must be released with ww_mutex_unlock.
+ */
+int __sched
+ww_mutex_trylock(struct ww_mutex *ww, struct ww_acquire_ctx *ctx)
+{
+	bool locked;
+
+	if (!ctx)
+		return mutex_trylock(&ww->base);
+
+#ifdef CONFIG_DEBUG_MUTEXES
+	DEBUG_LOCKS_WARN_ON(ww->base.magic != &ww->base);
+#endif
+
+	preempt_disable();
+	locked = __mutex_trylock(&ww->base);
+
+	if (locked) {
+		ww_mutex_set_context_fastpath(ww, ctx);
+		mutex_acquire_nest(&ww->base.dep_map, 0, 1, &ctx->dep_map, _RET_IP_);
+	}
+	preempt_enable();
+
+	return locked;
+}
+EXPORT_SYMBOL(ww_mutex_trylock);
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 void __sched
 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
diff --git a/kernel/locking/test-ww_mutex.c b/kernel/locking/test-ww_mutex.c
index 3e82f449b4ff..d63ac411f367 100644
--- a/kernel/locking/test-ww_mutex.c
+++ b/kernel/locking/test-ww_mutex.c
@@ -16,6 +16,15 @@
 static DEFINE_WD_CLASS(ww_class);
 struct workqueue_struct *wq;
 
+#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
+#define ww_acquire_init_noinject(a, b) do { \
+		ww_acquire_init((a), (b)); \
+		(a)->deadlock_inject_countdown = ~0U; \
+	} while (0)
+#else
+#define ww_acquire_init_noinject(a, b) ww_acquire_init((a), (b))
+#endif
+
 struct test_mutex {
 	struct work_struct work;
 	struct ww_mutex mutex;
@@ -36,7 +45,7 @@ static void test_mutex_work(struct work_struct *work)
 	wait_for_completion(&mtx->go);
 
 	if (mtx->flags & TEST_MTX_TRY) {
-		while (!ww_mutex_trylock(&mtx->mutex))
+		while (!ww_mutex_trylock(&mtx->mutex, NULL))
 			cond_resched();
 	} else {
 		ww_mutex_lock(&mtx->mutex, NULL);
@@ -109,19 +118,38 @@ static int test_mutex(void)
 	return 0;
 }
 
-static int test_aa(void)
+static int test_aa(bool trylock)
 {
 	struct ww_mutex mutex;
 	struct ww_acquire_ctx ctx;
 	int ret;
+	const char *from = trylock ? "trylock" : "lock";
 
 	ww_mutex_init(&mutex, &ww_class);
 	ww_acquire_init(&ctx, &ww_class);
 
-	ww_mutex_lock(&mutex, &ctx);
+	if (!trylock) {
+		ret = ww_mutex_lock(&mutex, &ctx);
+		if (ret) {
+			pr_err("%s: initial lock failed!\n", __func__);
+			goto out;
+		}
+	} else {
+		if (!ww_mutex_trylock(&mutex, &ctx)) {
+			pr_err("%s: initial trylock failed!\n", __func__);
+			goto out;
+		}
+	}
 
-	if (ww_mutex_trylock(&mutex))  {
-		pr_err("%s: trylocked itself!\n", __func__);
+	if (ww_mutex_trylock(&mutex, NULL))  {
+		pr_err("%s: trylocked itself without context from %s!\n", __func__, from);
+		ww_mutex_unlock(&mutex);
+		ret = -EINVAL;
+		goto out;
+	}
+
+	if (ww_mutex_trylock(&mutex, &ctx))  {
+		pr_err("%s: trylocked itself with context from %s!\n", __func__, from);
 		ww_mutex_unlock(&mutex);
 		ret = -EINVAL;
 		goto out;
@@ -129,17 +157,17 @@ static int test_aa(void)
 
 	ret = ww_mutex_lock(&mutex, &ctx);
 	if (ret != -EALREADY) {
-		pr_err("%s: missed deadlock for recursing, ret=%d\n",
-		       __func__, ret);
+		pr_err("%s: missed deadlock for recursing, ret=%d from %s\n",
+		       __func__, ret, from);
 		if (!ret)
 			ww_mutex_unlock(&mutex);
 		ret = -EINVAL;
 		goto out;
 	}
 
+	ww_mutex_unlock(&mutex);
 	ret = 0;
 out:
-	ww_mutex_unlock(&mutex);
 	ww_acquire_fini(&ctx);
 	return ret;
 }
@@ -150,7 +178,7 @@ struct test_abba {
 	struct ww_mutex b_mutex;
 	struct completion a_ready;
 	struct completion b_ready;
-	bool resolve;
+	bool resolve, trylock;
 	int result;
 };
 
@@ -160,8 +188,13 @@ static void test_abba_work(struct work_struct *work)
 	struct ww_acquire_ctx ctx;
 	int err;
 
-	ww_acquire_init(&ctx, &ww_class);
-	ww_mutex_lock(&abba->b_mutex, &ctx);
+	ww_acquire_init_noinject(&ctx, &ww_class);
+	if (!abba->trylock)
+		ww_mutex_lock(&abba->b_mutex, &ctx);
+	else
+		WARN_ON(!ww_mutex_trylock(&abba->b_mutex, &ctx));
+
+	WARN_ON(READ_ONCE(abba->b_mutex.ctx) != &ctx);
 
 	complete(&abba->b_ready);
 	wait_for_completion(&abba->a_ready);
@@ -181,7 +214,7 @@ static void test_abba_work(struct work_struct *work)
 	abba->result = err;
 }
 
-static int test_abba(bool resolve)
+static int test_abba(bool trylock, bool resolve)
 {
 	struct test_abba abba;
 	struct ww_acquire_ctx ctx;
@@ -192,12 +225,18 @@ static int test_abba(bool resolve)
 	INIT_WORK_ONSTACK(&abba.work, test_abba_work);
 	init_completion(&abba.a_ready);
 	init_completion(&abba.b_ready);
+	abba.trylock = trylock;
 	abba.resolve = resolve;
 
 	schedule_work(&abba.work);
 
-	ww_acquire_init(&ctx, &ww_class);
-	ww_mutex_lock(&abba.a_mutex, &ctx);
+	ww_acquire_init_noinject(&ctx, &ww_class);
+	if (!trylock)
+		ww_mutex_lock(&abba.a_mutex, &ctx);
+	else
+		WARN_ON(!ww_mutex_trylock(&abba.a_mutex, &ctx));
+
+	WARN_ON(READ_ONCE(abba.a_mutex.ctx) != &ctx);
 
 	complete(&abba.a_ready);
 	wait_for_completion(&abba.b_ready);
@@ -249,7 +288,7 @@ static void test_cycle_work(struct work_struct *work)
 	struct ww_acquire_ctx ctx;
 	int err, erra = 0;
 
-	ww_acquire_init(&ctx, &ww_class);
+	ww_acquire_init_noinject(&ctx, &ww_class);
 	ww_mutex_lock(&cycle->a_mutex, &ctx);
 
 	complete(cycle->a_signal);
@@ -581,7 +620,9 @@ static int stress(int nlocks, int nthreads, unsigned int flags)
 static int __init test_ww_mutex_init(void)
 {
 	int ncpus = num_online_cpus();
-	int ret;
+	int ret, i;
+
+	printk(KERN_INFO "Beginning ww mutex selftests\n");
 
 	wq = alloc_workqueue("test-ww_mutex", WQ_UNBOUND, 0);
 	if (!wq)
@@ -591,17 +632,19 @@ static int __init test_ww_mutex_init(void)
 	if (ret)
 		return ret;
 
-	ret = test_aa();
+	ret = test_aa(false);
 	if (ret)
 		return ret;
 
-	ret = test_abba(false);
+	ret = test_aa(true);
 	if (ret)
 		return ret;
 
-	ret = test_abba(true);
-	if (ret)
-		return ret;
+	for (i = 0; i < 4; i++) {
+		ret = test_abba(i & 1, i & 2);
+		if (ret)
+			return ret;
+	}
 
 	ret = test_cycle(ncpus);
 	if (ret)
@@ -619,6 +662,7 @@ static int __init test_ww_mutex_init(void)
 	if (ret)
 		return ret;
 
+	printk(KERN_INFO "All ww mutex selftests passed\n");
 	return 0;
 }
 
diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
index 161108e5d2fe..71652e1c397c 100644
--- a/lib/locking-selftest.c
+++ b/lib/locking-selftest.c
@@ -258,7 +258,7 @@ static void init_shared_classes(void)
 #define WWAF(x)			ww_acquire_fini(x)
 
 #define WWL(x, c)		ww_mutex_lock(x, c)
-#define WWT(x)			ww_mutex_trylock(x)
+#define WWT(x)			ww_mutex_trylock(x, NULL)
 #define WWL1(x)			ww_mutex_lock(x, NULL)
 #define WWU(x)			ww_mutex_unlock(x)
 
-- 
2.33.0


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