drm/atomic: Prepare drm_modeset_lock infrastructure for interruptible waiting, v2.

When we want to make drm_atomic_commit interruptible, there are a lot of
places that call the lock function, which we don't have control over.

Rather than trying to convert every single one, it's easier to toggle
interruptible waiting per acquire_ctx. If drm_modeset_acquire_init is
called with DRM_MODESET_ACQUIRE_INTERRUPTIBLE, then we will perform
interruptible waits in drm_modeset_lock and drm_modeset_backoff.

Changes since v1:
- Fix locking example in drm_modeset_lock.c to be compatible
  with interruptible waiting (xexaxo) and make it default.
  Uninterruptible waiting shouldn't happen except in corner cases,
  but the example will still apply if the flag is removed.
- Add drm_modeset_lock_single_interruptible() to documentation.
- Fix dead link to removed drm_modeset_lock_interruptible() in
  drm_modeset_lock().

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> #v1
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170912133749.6532-2-maarten.lankhorst@linux.intel.com
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

drivers/gpu/drm/drm_debugfs_crc.c

@@ -155,7 +155,7 @@ static int crtc_crc_open(struct inode *inode, struct file *filep)
 	int ret = 0;
 
 	if (drm_drv_uses_atomic_modeset(crtc->dev)) {
-		ret = drm_modeset_lock_interruptible(&crtc->mutex, NULL);
+		ret = drm_modeset_lock_single_interruptible(&crtc->mutex);
 		if (ret)
 			return ret;
 

drivers/gpu/drm/drm_modeset_lock.c

@@ -39,23 +39,28 @@
  *
  * The basic usage pattern is to::
  *
- *     drm_modeset_acquire_init(&ctx)
+ *     drm_modeset_acquire_init(ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE)
  *     retry:
  *     foreach (lock in random_ordered_set_of_locks) {
- *         ret = drm_modeset_lock(lock, &ctx)
+ *         ret = drm_modeset_lock(lock, ctx)
  *         if (ret == -EDEADLK) {
- *             drm_modeset_backoff(&ctx);
- *             goto retry;
+ *             ret = drm_modeset_backoff(ctx);
+ *             if (!ret)
+ *                 goto retry;
  *         }
+ *         if (ret)
+ *             goto out;
  *     }
  *     ... do stuff ...
- *     drm_modeset_drop_locks(&ctx);
- *     drm_modeset_acquire_fini(&ctx);
+ *     out:
+ *     drm_modeset_drop_locks(ctx);
+ *     drm_modeset_acquire_fini(ctx);
  *
  * If all that is needed is a single modeset lock, then the &struct
  * drm_modeset_acquire_ctx is not needed and the locking can be simplified
- * by passing a NULL instead of ctx in the drm_modeset_lock()
- * call and, when done, by calling drm_modeset_unlock().
+ * by passing a NULL instead of ctx in the drm_modeset_lock() call or
+ * calling  drm_modeset_lock_single_interruptible(). To unlock afterwards
+ * call drm_modeset_unlock().
  *
  * On top of these per-object locks using &ww_mutex there's also an overall
  * &drm_mode_config.mutex, for protecting everything else. Mostly this means
@@ -178,7 +183,11 @@ EXPORT_SYMBOL(drm_warn_on_modeset_not_all_locked);
 /**
  * drm_modeset_acquire_init - initialize acquire context
  * @ctx: the acquire context
- * @flags: for future
+ * @flags: 0 or %DRM_MODESET_ACQUIRE_INTERRUPTIBLE
+ *
+ * When passing %DRM_MODESET_ACQUIRE_INTERRUPTIBLE to @flags,
+ * all calls to drm_modeset_lock() will perform an interruptible
+ * wait.
  */
 void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
 		uint32_t flags)
@@ -186,6 +195,9 @@ void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
 	memset(ctx, 0, sizeof(*ctx));
 	ww_acquire_init(&ctx->ww_ctx, &crtc_ww_class);
 	INIT_LIST_HEAD(&ctx->locked);
+
+	if (flags & DRM_MODESET_ACQUIRE_INTERRUPTIBLE)
+		ctx->interruptible = true;
 }
 EXPORT_SYMBOL(drm_modeset_acquire_init);
 
@@ -261,8 +273,19 @@ static inline int modeset_lock(struct drm_modeset_lock *lock,
 	return ret;
 }
 
-static int modeset_backoff(struct drm_modeset_acquire_ctx *ctx,
-		bool interruptible)
+/**
+ * drm_modeset_backoff - deadlock avoidance backoff
+ * @ctx: the acquire context
+ *
+ * If deadlock is detected (ie. drm_modeset_lock() returns -EDEADLK),
+ * you must call this function to drop all currently held locks and
+ * block until the contended lock becomes available.
+ *
+ * This function returns 0 on success, or -ERESTARTSYS if this context
+ * is initialized with %DRM_MODESET_ACQUIRE_INTERRUPTIBLE and the
+ * wait has been interrupted.
+ */
+int drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx)
 {
 	struct drm_modeset_lock *contended = ctx->contended;
 
@@ -273,35 +296,10 @@ static int modeset_backoff(struct drm_modeset_acquire_ctx *ctx,
 
 	drm_modeset_drop_locks(ctx);
 
-	return modeset_lock(contended, ctx, interruptible, true);
-}
-
-/**
- * drm_modeset_backoff - deadlock avoidance backoff
- * @ctx: the acquire context
- *
- * If deadlock is detected (ie. drm_modeset_lock() returns -EDEADLK),
- * you must call this function to drop all currently held locks and
- * block until the contended lock becomes available.
- */
-void drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx)
-{
-	modeset_backoff(ctx, false);
+	return modeset_lock(contended, ctx, ctx->interruptible, true);
 }
 EXPORT_SYMBOL(drm_modeset_backoff);
 
-/**
- * drm_modeset_backoff_interruptible - deadlock avoidance backoff
- * @ctx: the acquire context
- *
- * Interruptible version of drm_modeset_backoff()
- */
-int drm_modeset_backoff_interruptible(struct drm_modeset_acquire_ctx *ctx)
-{
-	return modeset_backoff(ctx, true);
-}
-EXPORT_SYMBOL(drm_modeset_backoff_interruptible);
-
 /**
  * drm_modeset_lock_init - initialize lock
  * @lock: lock to init
@@ -324,14 +322,18 @@ EXPORT_SYMBOL(drm_modeset_lock_init);
  * deadlock scenario has been detected and it is an error to attempt
  * to take any more locks without first calling drm_modeset_backoff().
  *
+ * If the @ctx is not NULL and initialized with
+ * %DRM_MODESET_ACQUIRE_INTERRUPTIBLE, this function will fail with
+ * -ERESTARTSYS when interrupted.
+ *
  * If @ctx is NULL then the function call behaves like a normal,
- * non-nesting mutex_lock() call.
+ * uninterruptible non-nesting mutex_lock() call.
  */
 int drm_modeset_lock(struct drm_modeset_lock *lock,
 		struct drm_modeset_acquire_ctx *ctx)
 {
 	if (ctx)
-		return modeset_lock(lock, ctx, false, false);
+		return modeset_lock(lock, ctx, ctx->interruptible, false);
 
 	ww_mutex_lock(&lock->mutex, NULL);
 	return 0;
@@ -339,21 +341,19 @@ int drm_modeset_lock(struct drm_modeset_lock *lock,
 EXPORT_SYMBOL(drm_modeset_lock);
 
 /**
- * drm_modeset_lock_interruptible - take modeset lock
+ * drm_modeset_lock_single_interruptible - take a single modeset lock
  * @lock: lock to take
- * @ctx: acquire ctx
  *
- * Interruptible version of drm_modeset_lock()
+ * This function behaves as drm_modeset_lock() with a NULL context,
+ * but performs interruptible waits.
+ *
+ * This function returns 0 on success, or -ERESTARTSYS when interrupted.
  */
-int drm_modeset_lock_interruptible(struct drm_modeset_lock *lock,
-		struct drm_modeset_acquire_ctx *ctx)
+int drm_modeset_lock_single_interruptible(struct drm_modeset_lock *lock)
 {
-	if (ctx)
-		return modeset_lock(lock, ctx, true, false);
-
 	return ww_mutex_lock_interruptible(&lock->mutex, NULL);
 }
-EXPORT_SYMBOL(drm_modeset_lock_interruptible);
+EXPORT_SYMBOL(drm_modeset_lock_single_interruptible);
 
 /**
  * drm_modeset_unlock - drop modeset lock

include/drm/drm_modeset_lock.h

@@ -34,6 +34,7 @@ struct drm_modeset_lock;
  * @contended: used internally for -EDEADLK handling
  * @locked: list of held locks
  * @trylock_only: trylock mode used in atomic contexts/panic notifiers
+ * @interruptible: whether interruptible locking should be used.
  *
  * Each thread competing for a set of locks must use one acquire
  * ctx.  And if any lock fxn returns -EDEADLK, it must backoff and
@@ -59,6 +60,9 @@ struct drm_modeset_acquire_ctx {
 	 * Trylock mode, use only for panic handlers!
 	 */
 	bool trylock_only;
+
+	/* Perform interruptible waits on this context. */
+	bool interruptible;
 };
 
 /**
@@ -82,12 +86,13 @@ struct drm_modeset_lock {
 	struct list_head head;
 };
 
+#define DRM_MODESET_ACQUIRE_INTERRUPTIBLE BIT(0)
+
 void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
 		uint32_t flags);
 void drm_modeset_acquire_fini(struct drm_modeset_acquire_ctx *ctx);
 void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx);
-void drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx);
-int drm_modeset_backoff_interruptible(struct drm_modeset_acquire_ctx *ctx);
+int drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx);
 
 void drm_modeset_lock_init(struct drm_modeset_lock *lock);
 
@@ -111,8 +116,7 @@ static inline bool drm_modeset_is_locked(struct drm_modeset_lock *lock)
 
 int drm_modeset_lock(struct drm_modeset_lock *lock,
 		struct drm_modeset_acquire_ctx *ctx);
-int drm_modeset_lock_interruptible(struct drm_modeset_lock *lock,
-		struct drm_modeset_acquire_ctx *ctx);
+int __must_check drm_modeset_lock_single_interruptible(struct drm_modeset_lock *lock);
 void drm_modeset_unlock(struct drm_modeset_lock *lock);
 
 struct drm_device;