[PATCH v11 18/23] fs/resctrl: Call arch code for every mount

[Tony Luck]

Linux file system code provides no serialization of mount(2) system
calls. Without such serialization calling resctrl_arch_pre_mount() on
every mount before acquiring rdtgroup_mutex would open up many complex
races between mount and unmount operations.

Add resctrl_mount_lock to provide serialization and protect resctrl_mounted.

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
---
v11:
	New patch. Locking at file system level replaces v10 attempt
	at locking inside architecture resctrl_arch_pre_mount() and
	resctrl_arch_unmount()

 fs/resctrl/rdtgroup.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 3ac518ed2368..7f09d76dbf8c 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -30,6 +30,9 @@
 
 #include "internal.h"
 
+/* Mutex protecting resctrl_mounted and mount/unmount operations */
+static DEFINE_MUTEX(resctrl_mount_lock);
+
 /* Mutex to protect rdtgroup access. */
 DEFINE_MUTEX(rdtgroup_mutex);
 
@@ -3149,6 +3152,7 @@ static void resctrl_unmount(void)
 {
 	struct rdt_resource *r;
 
+	mutex_lock(&resctrl_mount_lock);
 	cpus_read_lock();
 	mutex_lock(&rdtgroup_mutex);
 
@@ -3166,6 +3170,8 @@ static void resctrl_unmount(void)
 	resctrl_mounted = false;
 	mutex_unlock(&rdtgroup_mutex);
 	cpus_read_unlock();
+	resctrl_arch_unmount();
+	mutex_unlock(&resctrl_mount_lock);
 }
 
 static int rdt_get_tree(struct fs_context *fc)
@@ -3177,24 +3183,27 @@ 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;
+		goto out_mount_unlock;
 	}
 
 	/* Avoid races from pending operations from a previous mount */
 	if (atomic_read(&rdtgroup_default.waitcount) != 0) {
 		ret = -EBUSY;
-		goto out;
+		goto out_mount_unlock;
 	}
 
+	resctrl_arch_pre_mount();
+
+	cpus_read_lock();
+	mutex_lock(&rdtgroup_mutex);
+
 	if (!resctrl_alloc_capable() && !resctrl_mon_capable()) {
 		ret = -EINVAL;
 		goto out;
@@ -3289,6 +3298,8 @@ static int rdt_get_tree(struct fs_context *fc)
 	mutex_unlock(&rdtgroup_mutex);
 	cpus_read_unlock();
 
+	mutex_unlock(&resctrl_mount_lock);
+
 	ret = kernfs_get_tree(fc);
 	/*
 	 * resctrl can only be mounted once, new superblock only expected
@@ -3297,7 +3308,8 @@ static int rdt_get_tree(struct fs_context *fc)
 	if (!ctx->kfc.new_sb_created)
 		resctrl_unmount();
 	kernfs_put(rdt_root_kn);
-	return ret;
+
+	return 0;
 
 out_mondata:
 	if (resctrl_mon_capable())
@@ -3318,8 +3330,12 @@ static int rdt_get_tree(struct fs_context *fc)
 out_root:
 	rdtgroup_destroy_root();
 out:
+	resctrl_arch_unmount();
 	mutex_unlock(&rdtgroup_mutex);
 	cpus_read_unlock();
+out_mount_unlock:
+	mutex_unlock(&resctrl_mount_lock);
+
 	return ret;
 }
 
-- 
2.55.0