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fs: don't create the private nullfs mount under namespace_sem

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init_mount_tree() mounts the mutable rootfs on top of nullfs via
LOCK_MOUNT_EXACT(). That declares a pinned mountpoint with a cleanup
attribute in the scope of the whole function so the nullfs root inode
lock and namespace_sem are only dropped when init_mount_tree() returns.

This became a problem when the private nullfs instance for kthreads was
added. kern_mount() allocates a new superblock and alloc_super() takes
the new s_umount with SINGLE_DEPTH_NESTING and then shrinker_mutex via
shrinker_alloc(). Doing that with namespace_sem held teaches lockdep the
dependency

    namespace_sem -> s_umount/1 -> shrinker_mutex

With CONFIG_SHRINKER_DEBUG shrinker_debugfs_rename() takes the debugfs
directory inode lock under shrinker_mutex every time a block device is
mounted and lock_mount_exact() takes namespace_sem under the inode lock
of the mountpoint for every mount. So mounting anything on debugfs,
e.g. the tracefs automount on /sys/kernel/debug/tracing, closes the
cycle:

    WARNING: possible circular locking dependency detected
    7.3.0-rc3+ #17 Not tainted
    ------------------------------------------------------
    rasdaemon/4449 is trying to acquire lock:
    (namespace_sem){++++}-{4:4}, at: lock_mount_exact+0x4c/0x308

    but task is already holding lock:
    (&sb->s_type->i_mutex_key#17){++++}-{4:4}, at: lock_mount_exact+0x3c/0x308

    which lock already depends on the new lock.
    ...
    Chain exists of:
      namespace_sem --> shrinker_mutex --> &sb->s_type->i_mutex_key#17

This can't actually deadlock. init_mount_tree() runs single-threaded
during early boot before any other task exists and nothing allocates a
superblock under namespace_sem after that. But lockdep can't know that
and disables itself for the rest of the boot.

Move mounting the rootfs on top of nullfs into a helper so the locks
are dropped when it returns.

Fixes: 32750c77e8 ("fs: start all kthreads in nullfs")
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Closes: https://lore.kernel.org/15174353-3f4a-a1ca-5bd1-ea2a4c77828e@huawei.com
Link: https://patch.msgid.link/20260917-atemtechnik-bleichen-befassen-9a57db01baf0@brauner
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-09-17 13:20:25 +02:00
commit 5179241521
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