[PATCH v2] fs/resctrl: Fix deadlock for errors during mount

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

Sashiko noticed[1] a deadlock in the resctrl mount code.

rdt_get_tree() acquires rdtgroup_mutex before calling kernfs_get_tree(). If
superblock setup fails inside kernfs_get_tree(), the VFS calls kill_sb on
the same thread before the call returns.  rdt_kill_sb() unconditionally
attempts to acquire rdtgroup_mutex and deadlock occurs.

Move the call to kernfs_get_tree() outside of locks.

If kernfs_get_tree() fails and ctx->kfc.new_sb_created is set, then rdt_kill_sb()
has already been called and no further cleanup is needed.

Fixes: 5ff193fbde20 ("x86/intel_rdt: Add basic resctrl filesystem support")
Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://sashiko.dev/#/patchset/20260429184858.36423-1-tony.luck%40intel.com [1]
---

Changes since v1:
Link: https://lore.kernel.org/all/20260501185612.14442-1-tony.luck@intel.com/

AI solution from Claude abandoned after human (Reinette) and AI (Sashiko)
reviews found it lacking.

Reinette: Feel free to claim full authorship of this fix since you
provided direction for every aspect. At minimum the "Suggested-by:"
should be "Co-authored-by:" (with your sign-off).

 fs/resctrl/rdtgroup.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 5dfdaa6f9d8f..6709b74bd655 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2855,10 +2855,6 @@ static int rdt_get_tree(struct fs_context *fc)
 	if (ret)
 		goto out_mondata;
 
-	ret = kernfs_get_tree(fc);
-	if (ret < 0)
-		goto out_psl;
-
 	if (resctrl_arch_alloc_capable())
 		resctrl_arch_enable_alloc();
 	if (resctrl_arch_mon_capable())
@@ -2874,10 +2870,26 @@ static int rdt_get_tree(struct fs_context *fc)
 						   RESCTRL_PICK_ANY_CPU);
 	}
 
-	goto out;
+	/* Release locks because kernfs_get_tree() may call rdt_kill_sb() */
+	mutex_unlock(&rdtgroup_mutex);
+	cpus_read_unlock();
+	ret = kernfs_get_tree(fc);
+	if (!ret || ctx->kfc.new_sb_created) {
+		/* mount succeeded, or failed and already cleaned up */
+		return ret;
+	}
+	cpus_read_lock();
+	mutex_lock(&rdtgroup_mutex);
+
+	if (resctrl_arch_alloc_capable())
+		resctrl_arch_disable_alloc();
+	if (resctrl_arch_mon_capable())
+		resctrl_arch_disable_mon();
+
+	resctrl_mounted = false;
 
-out_psl:
 	rdt_pseudo_lock_release();
+
 out_mondata:
 	if (resctrl_arch_mon_capable())
 		kernfs_remove(kn_mondata);
-- 
2.54.0