fs/resctrl: Fix deadlock on errors during mount

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

Since mount failure resulting from kernfs_get_tree() already calls the
resctrl fs unmount handler (rdt_kill_sb()) let both call the same helper
to make it clear both paths perform the same cleanup.

Call 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.

kernfs_get_tree() may set ctx->kfc.new_sb_created and then fail to obtain
an inode for the new kn, causing the rdt_kill_sb() path to run with one fewer
reference than required for the root to remain accessible in kernfs_kill_sb().
Add an extra hold on rdtgroup_default.kn to defend against this scenario
and ensure the root can be dereferenced safely from kernfs_kill_sb().

Dropping locks before kernfs_get_tree() creates a window where CPU hotplug
callbacks can race with the mount operation. Specifically, an online event
observing resctrl_mounted == true could concurrently append directories to
the unactivated kernfs tree, allocate mon_data structures, and arm background
workers.

This concurrency is safe because the mount has not yet returned to the VFS,
meaning userspace cannot interact with these transient files. If
kernfs_get_tree() subsequently fails, the standard resctrl_unmount() teardown
safely manages the concurrent modifications: any dynamically generated kernfs
nodes are removed, and the associated memory is freed. Any background
workers spawned by the hotplug event will naturally exit without re-arming
when they acquire rdtgroup_mutex and observe resctrl_mounted == false.

Fixes: 5ff193fbde20 ("x86/intel_rdt: Add basic resctrl filesystem support")
Closes: https://sashiko.dev/#/patchset/20260429184858.36423-1-tony.luck%40intel.com [1]
Reported-by: Sashiko <sashiko-bot@kernel.org>
Co-developed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Link: https://patch.msgid.link/fe701825d7f538a6bbac6732230004050300c93e.1783963505.git.reinette.chatre@intel.com

0 files changed, 0 insertions, 0 deletions