[PATCH v8 04/21] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point

[James Morse <james.morse@arm.com>]

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>
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>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
---
Changes since v7:
 * Moved kernfs cleanup functions to resctrl_fs_teardown(), and call this
   from rescrl_exit().
 * Added description of MPAM resetting the hardware to the commit message.

Changes since v6:
 * Added kdoc and comment to resctrl_exit().

Changes since v5:
 * Serialise rdtgroup_destroy_root() against umount().
 * Check rdtgroup_default.kn to protect against duplicate calls.
---
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 42 +++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index fdf2616c7ca0..3f9c37637d7e 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -3074,6 +3074,22 @@ 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;
@@ -3087,12 +3103,7 @@ 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())
@@ -4123,6 +4134,8 @@ 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;
 }
@@ -4416,11 +4429,26 @@ int __init resctrl_init(void)
 	return ret;
 }
 
+/**
+ * resctrl_exit() - Remove the resctrl filesystem and free resources.
+ *
+ * Called by the architecture code in response to a fatal error.
+ * Resctrl files and structures are removed from kernfs to prevent further
+ * configuration.
+ */
 void __exit resctrl_exit(void)
 {
+	mutex_lock(&rdtgroup_mutex);
+	resctrl_fs_teardown();
+	mutex_unlock(&rdtgroup_mutex);
+
 	debugfs_remove_recursive(debugfs_resctrl);
 	unregister_filesystem(&rdt_fs_type);
-	sysfs_remove_mount_point(fs_kobj, "resctrl");
+
+	/*
+	 * The sysfs mount point added by resctrl_init() is not removed so that
+	 * it can be used to umount resctrl.
+	 */
 
 	resctrl_mon_resource_exit();
 }
-- 
2.20.1