[PATCH v4 2/2] coresight: Fix possible deadlock in coresight_panic_cb

[Sean Anderson <sean.anderson@linux.dev>]

Panics can occur at any time, so taking locks may cause a deadlock (such
as if the panicking CPU held the lock).  coresight_panic_cb uses
bus_for_each_dev, but that calls bus_to_subsys which takes
bus_kset->list_lock.

Instead of registering a single panic notifier and iterating over
coresight devices, register a panic notifier for each coresight device
that requires it (letting the atomic notifier list handle iteration).
atomic_notifier_chain_unregister will just return -ENOENT if a notifier
block isn't on the list, so it's safe to call when we haven't registered
a notifier.

Fixes: 46006ceb5d02 ("coresight: core: Add provision for panic callbacks")
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
---

Changes in v4:
- Move the panic notifier into csdev and restore the panic sync API

Changes in v3:
- Rewrite patch to remove the panic sync API entirely

Changes in v2:
- Add a comment describing csdev_lock/list
- Consolidate list removal in coresight_device_release

 drivers/hwtracing/coresight/coresight-core.c | 67 ++++++++------------
 include/linux/coresight.h                    |  2 +
 2 files changed, 27 insertions(+), 42 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-core.c b/drivers/hwtracing/coresight/coresight-core.c
index 022c8384b98d..6dfb1198c16e 100644
--- a/drivers/hwtracing/coresight/coresight-core.c
+++ b/drivers/hwtracing/coresight/coresight-core.c
@@ -1046,8 +1046,11 @@ static void coresight_device_release(struct device *dev)
 {
 	struct coresight_device *csdev = to_coresight_device(dev);
 
-	fwnode_handle_put(csdev->dev.fwnode);
+	if (panic_ops(csdev))
+		atomic_notifier_chain_unregister(&panic_notifier_list,
+						 &csdev->panic_notifier);
 	free_percpu(csdev->perf_sink_id_map.cpu_map);
+	fwnode_handle_put(csdev->dev.fwnode);
 	kfree(csdev);
 }
 
@@ -1315,6 +1318,16 @@ void coresight_release_platform_data(struct coresight_device *csdev,
 		coresight_remove_conns_sysfs_group(csdev);
 }
 
+static int coresight_panic_notifier(struct notifier_block *nb,
+				    unsigned long action, void *data)
+{
+	struct coresight_device *csdev =
+		container_of(nb, struct coresight_device, panic_notifier);
+
+	panic_ops(csdev)->sync(csdev);
+	return NOTIFY_DONE;
+}
+
 struct coresight_device *coresight_register(struct coresight_desc *desc)
 {
 	int ret;
@@ -1357,6 +1370,17 @@ struct coresight_device *coresight_register(struct coresight_desc *desc)
 			goto err_out;
 		}
 	}
+
+	if (panic_ops(csdev)) {
+		csdev->panic_notifier.notifier_call = coresight_panic_notifier;
+		ret = atomic_notifier_chain_register(&panic_notifier_list,
+						     &csdev->panic_notifier);
+		if (ret) {
+			coresight_device_release(&csdev->dev);
+			goto err_out;
+		}
+	}
+
 	/*
 	 * Make sure the device registration and the connection fixup
 	 * are synchronised, so that we don't see uninitialised devices
@@ -1563,36 +1587,6 @@ const struct bus_type coresight_bustype = {
 	.name	= "coresight",
 };
 
-static int coresight_panic_sync(struct device *dev, void *data)
-{
-	int mode;
-	struct coresight_device *csdev;
-
-	/* Run through panic sync handlers for all enabled devices */
-	csdev = container_of(dev, struct coresight_device, dev);
-	mode = coresight_get_mode(csdev);
-
-	if ((mode == CS_MODE_SYSFS) || (mode == CS_MODE_PERF)) {
-		if (panic_ops(csdev))
-			panic_ops(csdev)->sync(csdev);
-	}
-
-	return 0;
-}
-
-static int coresight_panic_cb(struct notifier_block *self,
-			       unsigned long v, void *p)
-{
-	bus_for_each_dev(&coresight_bustype, NULL, NULL,
-				 coresight_panic_sync);
-
-	return 0;
-}
-
-static struct notifier_block coresight_notifier = {
-	.notifier_call = coresight_panic_cb,
-};
-
 static int __init coresight_init(void)
 {
 	int ret;
@@ -1605,20 +1599,11 @@ static int __init coresight_init(void)
 	if (ret)
 		goto exit_bus_unregister;
 
-	/* Register function to be called for panic */
-	ret = atomic_notifier_chain_register(&panic_notifier_list,
-					     &coresight_notifier);
-	if (ret)
-		goto exit_perf;
-
 	/* initialise the coresight syscfg API */
 	ret = cscfg_init();
 	if (!ret)
 		return 0;
 
-	atomic_notifier_chain_unregister(&panic_notifier_list,
-					     &coresight_notifier);
-exit_perf:
 	etm_perf_exit();
 exit_bus_unregister:
 	bus_unregister(&coresight_bustype);
@@ -1628,8 +1613,6 @@ static int __init coresight_init(void)
 static void __exit coresight_exit(void)
 {
 	cscfg_exit();
-	atomic_notifier_chain_unregister(&panic_notifier_list,
-					     &coresight_notifier);
 	etm_perf_exit();
 	bus_unregister(&coresight_bustype);
 }
diff --git a/include/linux/coresight.h b/include/linux/coresight.h
index 4ac65c68bbf4..a7aaf9d3d01d 100644
--- a/include/linux/coresight.h
+++ b/include/linux/coresight.h
@@ -280,6 +280,7 @@ struct coresight_trace_id_map {
  * @config_csdev_list:  List of system configurations added to the device.
  * @cscfg_csdev_lock:	Protect the lists of configurations and features.
  * @active_cscfg_ctxt:  Context information for current active system configuration.
+ * @panic_notifier: Notifier block used to clean up during a panic
  */
 struct coresight_device {
 	struct coresight_platform_data *pdata;
@@ -304,6 +305,7 @@ struct coresight_device {
 	struct list_head config_csdev_list;
 	raw_spinlock_t cscfg_csdev_lock;
 	void *active_cscfg_ctxt;
+	struct notifier_block panic_notifier;
 };
 
 /*
-- 
2.35.1.1320.gc452695387.dirty