[RFC PATCH v5 6/7] fs/resctrl: Call architecture hooks for every mount/unmount

[Tony Luck <tony.luck@intel.com>]

Architecture code for AET needs to acquire INTEL_PMT_TELEMETRY module
references on mount and release them on unmount.

Add hooks for every mount/unmount of the resctrl file system so that
architecture code can allocate on mount and free on unmount.

Add resctrl_mount_lock to protect mount/unmount operations and write access
to "resctrl_mounted". This simplifies the code flow around the repeat mount
with -EBUSY return to avoid calling resctrl_arch_pre_mount() when the file
system is mounted.

Signed-off-by: Tony Luck <tony.luck@intel.com>
---
 include/linux/resctrl.h |  9 ++++++---
 fs/resctrl/rdtgroup.c   | 35 ++++++++++++++++++++++++++++-------
 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index f3eb6dfa61d4..f3da533e182c 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -551,12 +551,15 @@ void resctrl_online_cpu(unsigned int cpu);
 void resctrl_offline_cpu(unsigned int cpu);
 
 /*
- * Architecture hook called at beginning of first file system mount attempt.
- * No locks are held.
+ * Architecture hook called before attempting to mount the file system.
+ * Called with just resctrl_mount_lock held.
  */
 void resctrl_arch_pre_mount(void);
 
-/* Called to report unmount. */
+/*
+ * Architecture hook called when mount fails, or on unmount.
+ * Called with just resctrl_mount_lock held.
+ */
 void resctrl_arch_unmount(void);
 
 /**
diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 5da305bd36c9..444bc359e4a7 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -18,7 +18,6 @@
 #include <linux/fs_parser.h>
 #include <linux/sysfs.h>
 #include <linux/kernfs.h>
-#include <linux/once.h>
 #include <linux/resctrl.h>
 #include <linux/seq_buf.h>
 #include <linux/seq_file.h>
@@ -48,7 +47,14 @@ LIST_HEAD(resctrl_schema_all);
  */
 static LIST_HEAD(mon_data_kn_priv_list);
 
-/* The filesystem can only be mounted once. */
+/* Mutex protecting mount/unmount operations */
+static DEFINE_MUTEX(resctrl_mount_lock);
+
+/*
+ * The filesystem can only be mounted once.
+ * Updated with both rdtgroup_mutex and resctrl_mount_lock held.
+ * Safe to read while holding either rdtgroup_mutex or resctrl_mount_lock.
+ */
 bool resctrl_mounted;
 
 /* Kernel fs node for "info" directory under root */
@@ -2788,18 +2794,21 @@ static int rdt_get_tree(struct fs_context *fc)
 	struct rdt_resource *r;
 	int ret;
 
-	DO_ONCE_SLEEPABLE(resctrl_arch_pre_mount);
+	mutex_lock(&resctrl_mount_lock);
 
-	cpus_read_lock();
-	mutex_lock(&rdtgroup_mutex);
 	/*
 	 * resctrl file system can only be mounted once.
 	 */
 	if (resctrl_mounted) {
-		ret = -EBUSY;
-		goto out;
+		mutex_unlock(&resctrl_mount_lock);
+		return -EBUSY;
 	}
 
+	resctrl_arch_pre_mount();
+
+	cpus_read_lock();
+	mutex_lock(&rdtgroup_mutex);
+
 	ret = setup_rmid_lru_list();
 	if (ret)
 		goto out;
@@ -2897,6 +2906,12 @@ static int rdt_get_tree(struct fs_context *fc)
 	rdt_last_cmd_clear();
 	mutex_unlock(&rdtgroup_mutex);
 	cpus_read_unlock();
+
+	if (ret)
+		resctrl_arch_unmount();
+
+	mutex_unlock(&resctrl_mount_lock);
+
 	return ret;
 }
 
@@ -3169,6 +3184,8 @@ static void rdt_kill_sb(struct super_block *sb)
 {
 	struct rdt_resource *r;
 
+	mutex_lock(&resctrl_mount_lock);
+
 	cpus_read_lock();
 	mutex_lock(&rdtgroup_mutex);
 
@@ -3187,6 +3204,10 @@ static void rdt_kill_sb(struct super_block *sb)
 	kernfs_kill_sb(sb);
 	mutex_unlock(&rdtgroup_mutex);
 	cpus_read_unlock();
+
+	resctrl_arch_unmount();
+
+	mutex_unlock(&resctrl_mount_lock);
 }
 
 static struct file_system_type rdt_fs_type = {
-- 
2.53.0