x86/resctrl: Resctrl_exit() teardown resctrl but leave the mount point

resctrl_exit() was intended for use when the 'resctrl' module was unloaded.
resctrl can't be built as a module, and the kernfs helpers are not exported so
this is unlikely to change. MPAM has an error interrupt which indicates the
MPAM driver has gone haywire. Should this occur tasks could run with the wrong
control values, leading to bad performance for important tasks.  In this
scenario the MPAM driver will reset the hardware, but it needs a way to tell
resctrl that no further configuration should be attempted.

In particular, moving tasks between control or monitor groups does not
interact with the architecture code, so there is no opportunity for the arch
code to indicate that the hardware is no-longer functioning.

Using resctrl_exit() for this leaves the system in a funny state as resctrl is
still mounted, but cannot be un-mounted because the sysfs directory that is
typically used has been removed. Dave Martin suggests this may cause systemd
trouble in the future as not all filesystems can be unmounted.

Add calls to remove all the files and directories in resctrl, and remove the
sysfs_remove_mount_point() call that leaves the system in a funny state. When
triggered, this causes all the resctrl files to disappear. resctrl can be
unmounted, but not mounted again.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
Tested-by: Fenghua Yu <fenghuay@nvidia.com>
Tested-by: Carl Worth <carl@os.amperecomputing.com> # arm64
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Tested-by: Peter Newman <peternewman@google.com>
Tested-by: Amit Singh Tomar <amitsinght@marvell.com> # arm64
Tested-by: Shanker Donthineni <sdonthineni@nvidia.com> # arm64
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/20250515165855.31452-9-james.morse@arm.com

Author: James Morse

arch/x86/kernel/cpu/resctrl/rdtgroup.c

@@ -3078,6 +3078,22 @@ static void rmdir_all_sub(void)
 	kernfs_remove(kn_mondata);
 }
 
+static void resctrl_fs_teardown(void)
+{
+	lockdep_assert_held(&rdtgroup_mutex);
+
+	/* Cleared by rdtgroup_destroy_root() */
+	if (!rdtgroup_default.kn)
+		return;
+
+	rmdir_all_sub();
+	rdt_pseudo_lock_release();
+	rdtgroup_default.mode = RDT_MODE_SHAREABLE;
+	closid_exit();
+	schemata_list_destroy();
+	rdtgroup_destroy_root();
+}
+
 static void rdt_kill_sb(struct super_block *sb)
 {
 	struct rdt_resource *r;
@@ -3091,12 +3107,7 @@ static void rdt_kill_sb(struct super_block *sb)
 	for_each_alloc_capable_rdt_resource(r)
 		resctrl_arch_reset_all_ctrls(r);
 
-	rmdir_all_sub();
-	rdt_pseudo_lock_release();
-	rdtgroup_default.mode = RDT_MODE_SHAREABLE;
-	closid_exit();
-	schemata_list_destroy();
-	rdtgroup_destroy_root();
+	resctrl_fs_teardown();
 	if (resctrl_arch_alloc_capable())
 		resctrl_arch_disable_alloc();
 	if (resctrl_arch_mon_capable())
@@ -4127,6 +4138,8 @@ static int rdtgroup_setup_root(struct rdt_fs_context *ctx)
 
 static void rdtgroup_destroy_root(void)
 {
+	lockdep_assert_held(&rdtgroup_mutex);
+
 	kernfs_destroy_root(rdt_root);
 	rdtgroup_default.kn = NULL;
 }
@@ -4441,23 +4454,42 @@ static bool __exit resctrl_online_domains_exist(void)
 	return false;
 }
 
-/*
+/**
  * resctrl_exit() - Remove the resctrl filesystem and free resources.
  *
+ * Called by the architecture code in response to a fatal error.
+ * Removes resctrl files and structures from kernfs to prevent further
+ * configuration.
+ *
  * When called by the architecture code, all CPUs and resctrl domains must be
  * offline. This ensures the limbo and overflow handlers are not scheduled to
  * run, meaning the data structures they access can be freed by
  * resctrl_mon_resource_exit().
+ *
+ * After resctrl_exit() returns, the architecture code should return an
+ * error from all resctrl_arch_ functions that can do this.
+ * resctrl_arch_get_resource() must continue to return struct rdt_resources
+ * with the correct rid field to ensure the filesystem can be unmounted.
  */
 void __exit resctrl_exit(void)
 {
 	cpus_read_lock();
 	WARN_ON_ONCE(resctrl_online_domains_exist());
+
+	mutex_lock(&rdtgroup_mutex);
+	resctrl_fs_teardown();
+	mutex_unlock(&rdtgroup_mutex);
+
 	cpus_read_unlock();
 
 	debugfs_remove_recursive(debugfs_resctrl);
+	debugfs_resctrl = NULL;
 	unregister_filesystem(&rdt_fs_type);
-	sysfs_remove_mount_point(fs_kobj, "resctrl");
+
+	/*
+	 * Do not remove the sysfs mount point added by resctrl_init() so that
+	 * it can be used to umount resctrl.
+	 */
 
 	resctrl_mon_resource_exit();
 }