[PATCH v1 09/11] coresight: Take hotplug lock in enable_source_store() for Sysfs mode

[Leo Yan]

The hotplug lock is acquired and released in the etm4_disable_sysfs()
function, which is a low-level function located in the ETM4 driver.
This prevents us from a new solution for hotplug.

Firstly, hotplug callbacks cannot invoke etm4_disable_sysfs() to disable
the source; otherwise, a deadlock issue occurs.  The reason is that, in
the hotplug flow, the kernel acquires the hotplug lock before calling
callbacks.  Subsequently, if coresight_disable_source() is invoked and
it calls etm4_disable_sysfs(), the hotplug lock will be acquired twice,
leading to a double lock issue.

Secondly, when hotplugging a CPU on or off, if we want to manipulate all
components on a path attached to the CPU, we need to maintain atomicity
for the entire path.  Otherwise, a race condition may occur with users
setting the same path via the Sysfs knobs, ultimately causing mess
states in CoreSight components.

This patch moves the hotplug locking from etm4_disable_sysfs() into
enable_source_store().  As a result, when users control the Sysfs knobs,
the whole flow is protected by hotplug locking, ensuring it is mutual
exclusive with hotplug callbacks.

Note, the paired function etm4_enable_sysfs() does not use hotplug
locking, which is why this patch does not modify it.

Signed-off-by: Leo Yan <leo.yan at arm.com>
---
 drivers/hwtracing/coresight/coresight-etm4x-core.c |  8 --------
 drivers/hwtracing/coresight/coresight-sysfs.c      | 12 +++++++++++-
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index d5fd9e58a962..e5a7c0dd7f8e 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -1032,13 +1032,6 @@ static void etm4_disable_sysfs(struct coresight_device *csdev)
 {
 	struct etmv4_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
 
-	/*
-	 * Taking hotplug lock here protects from clocks getting disabled
-	 * with tracing being left on (crash scenario) if user disable occurs
-	 * after cpu online mask indicates the cpu is offline but before the
-	 * DYING hotplug callback is serviced by the ETM driver.
-	 */
-	cpus_read_lock();
 	raw_spin_lock(&drvdata->spinlock);
 
 	/*
@@ -1048,7 +1041,6 @@ static void etm4_disable_sysfs(struct coresight_device *csdev)
 	smp_call_function_single(drvdata->cpu, etm4_disable_hw, drvdata, 1);
 
 	raw_spin_unlock(&drvdata->spinlock);
-	cpus_read_unlock();
 
 	/*
 	 * we only release trace IDs when resetting sysfs.
diff --git a/drivers/hwtracing/coresight/coresight-sysfs.c b/drivers/hwtracing/coresight/coresight-sysfs.c
index feadaf065b53..ea839a5e601b 100644
--- a/drivers/hwtracing/coresight/coresight-sysfs.c
+++ b/drivers/hwtracing/coresight/coresight-sysfs.c
@@ -359,14 +359,24 @@ static ssize_t enable_source_store(struct device *dev,
 	if (ret)
 		return ret;
 
+	/*
+	 * CoreSight hotplug callbacks in core layer control a activated path
+	 * from its source to sink. Taking hotplug lock here protects a race
+	 * condition with hotplug callbacks.
+	 */
+	cpus_read_lock();
+
 	if (val) {
 		ret = coresight_enable_sysfs(csdev);
-		if (ret)
+		if (ret) {
+			cpus_read_unlock();
 			return ret;
+		}
 	} else {
 		coresight_disable_sysfs(csdev);
 	}
 
+	cpus_read_unlock();
 	return size;
 }
 static DEVICE_ATTR_RW(enable_source);
-- 
2.34.1