stm class: Fix a race in unlinking

There is a window in stm_source_link_drop(), during which the source's
link may change before locks are acquired. When this happens, it throws
a warning, since this is not an expected scenario.

This patch handles the race in such a way that if the link appears to
have changed by the time we took the locks, it will release them and
repeat the whole unlinking procedure from the beginning, unless the
other contender beat us to it.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/stm/core.c

@@ -694,18 +694,26 @@ int stm_register_device(struct device *parent, struct stm_data *stm_data,
 }
 EXPORT_SYMBOL_GPL(stm_register_device);
 
-static void __stm_source_link_drop(struct stm_source_device *src,
-				   struct stm_device *stm);
+static int __stm_source_link_drop(struct stm_source_device *src,
+				  struct stm_device *stm);
 
 void stm_unregister_device(struct stm_data *stm_data)
 {
 	struct stm_device *stm = stm_data->stm;
 	struct stm_source_device *src, *iter;
-	int i;
+	int i, ret;
 
 	mutex_lock(&stm->link_mutex);
 	list_for_each_entry_safe(src, iter, &stm->link_list, link_entry) {
-		__stm_source_link_drop(src, stm);
+		ret = __stm_source_link_drop(src, stm);
+		/*
+		 * src <-> stm link must not change under the same
+		 * stm::link_mutex, so complain loudly if it has;
+		 * also in this situation ret!=0 means this src is
+		 * not connected to this stm and it should be otherwise
+		 * safe to proceed with the tear-down of stm.
+		 */
+		WARN_ON_ONCE(ret);
 	}
 	mutex_unlock(&stm->link_mutex);
 
@@ -824,22 +832,28 @@ static int stm_source_link_add(struct stm_source_device *src,
  *
  * Caller must hold stm::link_mutex.
  */
-static void __stm_source_link_drop(struct stm_source_device *src,
-				   struct stm_device *stm)
+static int __stm_source_link_drop(struct stm_source_device *src,
+				  struct stm_device *stm)
 {
 	struct stm_device *link;
+	int ret = 0;
 
 	lockdep_assert_held(&stm->link_mutex);
 
-	if (src->data->unlink)
-		src->data->unlink(src->data);
-
 	/* for stm::link_list modification, we hold both mutex and spinlock */
 	spin_lock(&stm->link_lock);
 	spin_lock(&src->link_lock);
 	link = srcu_dereference_check(src->link, &stm_source_srcu, 1);
-	if (WARN_ON_ONCE(link != stm))
+
+	/*
+	 * The linked device may have changed since we last looked, because
+	 * we weren't holding the src::link_lock back then; if this is the
+	 * case, tell the caller to retry.
+	 */
+	if (link != stm) {
+		ret = -EAGAIN;
 		goto unlock;
+	}
 
 	stm_output_free(link, &src->output);
 	list_del_init(&src->link_entry);
@@ -850,6 +864,11 @@ static void __stm_source_link_drop(struct stm_source_device *src,
 unlock:
 	spin_unlock(&src->link_lock);
 	spin_unlock(&stm->link_lock);
+
+	if (!ret && src->data->unlink)
+		src->data->unlink(src->data);
+
+	return ret;
 }
 
 /**
@@ -865,18 +884,29 @@ static void __stm_source_link_drop(struct stm_source_device *src,
 static void stm_source_link_drop(struct stm_source_device *src)
 {
 	struct stm_device *stm;
-	int idx;
+	int idx, ret;
 
+retry:
 	idx = srcu_read_lock(&stm_source_srcu);
+	/*
+	 * The stm device will be valid for the duration of this
+	 * read section, but the link may change before we grab
+	 * the src::link_lock in __stm_source_link_drop().
+	 */
 	stm = srcu_dereference(src->link, &stm_source_srcu);
 
+	ret = 0;
 	if (stm) {
 		mutex_lock(&stm->link_mutex);
-		__stm_source_link_drop(src, stm);
+		ret = __stm_source_link_drop(src, stm);
 		mutex_unlock(&stm->link_mutex);
 	}
 
 	srcu_read_unlock(&stm_source_srcu, idx);
+
+	/* if it did change, retry */
+	if (ret == -EAGAIN)
+		goto retry;
 }
 
 static ssize_t stm_source_link_show(struct device *dev,